Several times last year when I put the car (manual) into first gear it would not move, I would try again and with effort it would go, I then discovered that by pshifting it into second gear when this happened the car would go. I have asked my Volvo mechanic about this a couple of times but he had no ideas about what it could be. For the last few months it has been fine, but these symptoms have returned twice in the last couple of days. When I do start off in the car I do so gently, doing a fast start can sometimes make the car "buck" slightly in the back end, perhaps these two things are related? The car is well maintained and runs very well apart from the above.

Is it possible that you're actually not getting the shifter into first gear when this happens? I mean, does it seem like you are in neutral- you let the clutch out, the engine doesn't stall, it revs up and you just sit there?
If this is the case, then I would think you've got a problem with the shift linkage on the back of the transmission. This could be due to the pivot for the shifter being worn out, or possibly the transmission mount is shot, so the shifter no longer lines up nicely with the hole in the body. Also, the entire shift linkage bracket can come loose from the back end of the transmission.

Have the mechanic check the mount and the entire shift linkage for problems.

Alternately, if the car seems like it refuses to roll freely, even though the clutch is being let out, it could be something like the parking brake is sticking on, or a brake hose is restricting flow back to the master cylinder.
--
Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K

Thanks Rob,
those seem like really good possibilities to me. The car will be going in very shortly to have some other work done and I will ask for this to be checked out also, and will report back. The car is my daily driver and is just coming up to 180,000, so should have a lot of life left in it yet.