VCR tape playback not good

A common complaint about videotape is that over time, playback becomes unstable and often deteriorates to the point that the tape becomes unplayable or that the tape works on one VCR, but not another. All of these problems can be traced to tape path alignment in the VCR and damage to the tape itself. I will address each issue separately.
First you need to understand a little about the tape media. When you record on VHS videotape, the recorder lays down three (four for stereo sound) magnetic tracks on the ½ inch wide tape. Analog audio is recorded along the top edge as a thin horizontal stripe (or parallel stripes for stereo) for the entire length of the recording. In the center of the tape, video is recorded as diagonal parallel stripes by two or four heads that rotate on a drum at 360 RPM. On the bottom edge (the most vulnerable place for damage to occur) there is another horizontal track that is critical to playback. It is the sync track and it's purpose is provide the VCR with the feedback it requires to maintain the tape speed within extremely tight parameters. If the tape does not move at a precise speed, the picture and sound can become unstable to the point that it will be unusable.
As you record, a fixed frequency sine wave is recorded on the sync track. If during recording, the tape speed increases or decreases, it will be reflected on the sync track. During playback, the VCR's circuitry senses the small millisecond-to-millisecond speed fluctuations of the tape movement over the heads and adjusts the speed so the tape speed always matches the speed at which it was recorded. If the sync track is ever damaged, your tape can become useless.
Unfortunately, videotape is a very delicate media. It is easily damaged and once damaged, it usually cannot be repaired. The most common failure is due to tape stretch. Videotape is not very elastic. Anything more then slight tension during use can stretch the tape to the point that it cannot rebound. If the sync track is stretched, the recorded frequency will change and the VCR will react accordingly by making incorrect speed corrections resulting in picture and sound disruptions. Your recorder has tensioning arms that control tape tension as it moves through the system. If the tension in the tape path is incorrectly adjusted, the tape will be damaged as it is recorded or played.
Improper storage is the other common cause of edge damage. If you lay your cassettes flat for prolonged periods, the weight of the tape above will press against the sync track and can damage it. Always store your tapes vertically. Extremes in temperature and humidity can cause stretch. Store tapes in a cool dry place. Also, periodically restack your tapes. To restack a tape, put it in your recorder and fast-forward it to the end and back to the beginning. That will redistribute the tension on the tape. After playing a tape, always restack it once or twice. I have a large collection; over 1,000 tapes. I cannot restack them all on a regular basis. But when I play them, I take that as an opportunity to restack them.
Now that you understand some basics about the media, I will now turn you attention to your VCR. As I hope you are starting to understand, proper playback requires precise alignment of many moving elements. The two broad categories are tape path alignment and head alignment. None of this is a do it yourself job. In addition to specialized electronic test equipment, VCR alignment requires specialized tools along with expensive custom made for the brand and model alignment jigs along with (again expensive) alignment tapes. The alignment tape is important beyond the obvious. In order for a tape recorded on one machine to play properly on another, the two machines must have matching alignment. VCR manufacturers record their own alignment tapes using precisely and frequently aligned recorders. The tapes are used to align a specified number of VCRs and then discarded because each time a tape is used it wares. Since there is a uniform standard for these tapes, a tape recorded on one brand of VCR should play on another. That is the theory.
Now for the real world; the consistent interchangeability we all wish for is hard to maintain. Consider this. The tolerances that must be maintained at every point in all of the processes relative to VCRs and tapes are very close to what is possible; little room for slight variations. Since the tolerance for one part of the process may accumulate with another part of the process either mathematically positively or negatively, it is often the case that each individual part of the chain is within tolerance, but the sum total is out of tolerance.
What in plane language does this all mean? Two alignment tapes made on the same machine will be different. Tapes from different manufactures will be different. The tape you local technician or factory service center uses will have inconsistencies. The net effect is that two seemingly properly aligned machines may not be able to properly play each other's tapes.
I do not mean to suggest that you should not have your VCR properly aligned and maintained by a professional. My point is that videotape is an old technology with flaws that could not fully be overcome in the time frame that it would have been profitable for manufacturers to do so. That is why we have moved on to digital technologies. My advice is to enjoy your videotapes while you can. They will not last forever. If you have important tapes, transfer them to digital media to protect your memories as soon as possible.