Ok, if the first suggestion is no good, are you abslutely sure the sliding yoke on the propshaft is ok. It is unusual for them to get really bad but if they do they can lock and unlock as the transmission winds up. certainly as the car squats down when you accelerate the length of the prop will change, which is why the slider is there.

Also when the UJ's were changed did you check all of them are moving freely? As you put the UJ's in it is possible to get the bearing cap too far in. I usually put each eye of the prop on the workbench and tap the side of the prop with a leather mallet to settle each cap snugly against the circlip.

Are you sure the prop belongs to the car. Volvo subtly changed the flanges and if you have a mismatch the prop will fit but has to be forced with the bolts to lie snugly against the drive flanges on the 'box and axle. Smilarly there has to be free movement on the yoke, ie when the car is at rest the male part of the prop must not be fully home, it should be able to slide both in and out, otherwise he yoke can lock up.

Regards