Are you replacing all the fluid?
On Volvos equipped with automatic transmissons, you should alternate between taking off the pan to service the transmission and flushing it by using the cooling lines. The AW70/AW71 automatic transmission used on many Volvos from approximately 1982 holds 7.8 US qts. When servicing the filter screen, only about 3.5 US qts. can be drained. The rest remains in the torque converter and control systems. This means that less than 1/2 of the oil is changed when servicing is done by removing the pan.

We recommend disconnecting the cooling lines to flush the system to get new fluid into all areas of the transmisson. This requires a bucket to catch the old fluid on the outlet side, and a suitable pump on the inlet side to pump fluid into the transmisson as old fluid is being pumped out.

You may want to do the pan-style service yourself and have the flush done by a shop that has a pump. We have heard claims of up to twice the longevity for transmissons maintained in this manner.