Hope this rather late answer is still of some use.

There is an easier way (perhaps the intended way) to check ATF level in the B6304 Volvos. The top end of the yellow dipstick is designed to accept a ½ inch drive extension and a long extension with a wobble head would make the operation a piece of cake. Below the said receptacle are four ears or petals, for want of a better term. When one of the ears is in line with the retention spring clip on the filler tube, the dipstick is secured in place by the clip. That’s how one wants it to be normally. Rotating the dipstick 45% either way would disengage the dipstick from the clip and it can now be pulled up freely.

The only way to accurately identify a transmission problem is by pulling the error code(s). A couple of years ago I had a similar problem bugging me for months. Replacing the S1 and S2 shift solenoids, as indicated by the codes, solved the problem. I also replaced the PNP switch not because it was at fault but because it was 17 years old and I had an easy way to do it without disconnecting any transmission lines. The physical work is relatively easy. The difficult part is the research to source replacement parts. It would be near impossible to just buy the solenoids from Volvo sources but they are actually made by Rostra and very affordably priced.