I have a 2004 XC70 with 128,000 miles on it. When driving, the transmission intermittently clunks into a high gear and stays there. The gear indicator on the dash switches from "D" to "-" when this happens. Turning the car off and then back on resets the transmission and the dash indicator goes back to "D" for a while. Recently, the transmission oil and filter were exchanged but didn't correct the problem. What could be the cause?

SOURCE: 1994 Volvo 850 2.4L Auto transmission

Clear your engine and transmission codes, its a small box in the pass side engine compartment right behind the headlight. Search "Bay 13 Volvo" and that has all the information to check and clear the codes. It also has a list of what the codes mean to help you.

SOURCE: 1996 Volvo Oil Issue

Change the oil and filter.  I had this same thing happen; the sludge reminded me of chocolate mousse...  The mechanic who takes care of my car pointed out it had been a while since my last oil filter change.  Over time the filter gets clogged, and the engine basically whips up it's own oil - a la mousse.  A more expensive possibility is that the PCV valve is clogged and needs to be changed.  Again, in this instance the engine can't breathe properly, so it not only whips the oil up, but starts to shoot it out the dipstick hole, sort of like when you have to breath through your mouth when your nose gets clogged up because of a cold.  Didn't seem to injure the car, or freak out the mechanic.  Best of luck.

SOURCE: auto transmission will not go into high gear.

Just went thru this on my 93 240. There's a solenoid inside the glove box, on the left side that needed replacing, with age it started getting fussy, and one day I had overdrive, and the same afternoon I didn't, then next day it was fine, then it wasn't, and so on...Most annoying driving around in 3rd all day....Anyways, the part was about 75$ at a dealer, and installation was about 5 minutes...Solved the problem just fine...Also have your mechanic check the shift cable....it needed changing at the same time as the solenoid...jda/boston

SOURCE: 2001 Volvo V40 arrow warning light

Most likely the transmission range switch is failing.
Have someone check the transmission computer for codes.

SOURCE: speedometer erratic/intermittent

I don't know what is causing this problem, but I know phil140701's answer is completely wrong on two counts. First, 740 Volvos starting at least in 1986 have electronic speedometers and no cable. There is an electronic sensor on the differential which sends a signal to the speedometer cluster. Second, if it were strictly mechanical with a cable then the voltage regulator could have no effect on it at all.