1991 Volvo 940 transmission slipping

I have a 1993 940 Turbo GL with 270,000 on the original transmission. Transmission shop told me I had to have it re-built at 125,000 [$900+ estimate] ... it was slipping [at stop light, start to go but the engine only revs ... let off gas and then it slowly goes into gear. Slipped a little on 2nd and third gears also.] Drained all the transmission fluid [dropped the pan, , to make sure it was completely empty] replaced the pan gasket, the filter, added all new trans fluid ... and it has been 140,000 + miles on that change. Time to do it again I think. It is pretty sensative to the level of the fluid so don't add too much.