I have a 1986 Suzuki Intruder VS700 - after I ride for about 10 to 15 miles the clutch lever loses compression and I can't shift gears. If I let it stand for about twenty minutes the lever regains enough pressure but it will last for only another 5 - 10 miles before it happens again. Could it be as simple as changing the fluid?

More information please. What EXACTLY is it doing? Slack clutch lever? Stuck clutch lever? Crunching or grinding as you try to shift? Grabbing and stalling as it selects the gear?
Start at the clutch lever. Check it is pulling the cable then check at the transmission end that the cable is connected and moving with the lever. Next check the lever on the actual transmission. Does it seem too tight or too slack? These will give clues as to where the issue could be.
It's a 37 year old bike so is it the original Clutch? Even motorcycle clutches wear out.

Probably caused by a clutch fault - failing to clear...

Change the fluid in the clutch reservoir,bleed out the remaining fluid.(keep it full as you go) Should be dot 4 fluid not dot 3. U may need to rebuild the master cylinder or replace it.

THE CLUTCH PLATES COULD BE WORN ALSO THERE ARE ADJUSTMENTS BY THE CLUTCH LEVER TURN IT COUNTER CLOCK WISE TO TAKE SLACK OUT OF THE CLUTCH CABLE

shift fork in trans is not engaging need to let someone open trans and sovle the issue those trans are not hard to fix but if you dont know how the shift fork needs to be to fuction seek pro help