450 SL transmission problem

I recently purchase a 1977 450SL with 170,000 well maintained miles. On my way home with the car from Kansas City to Arkansas, I noticed that the car stopped upshifting from 1-2. It will shift by letting off the accelerator. Also, on the 2-1 down thift there is an audible thump in the transmission. It goes into reverse fine though. The transmission fluid is brown so I ordered a gasket and filter set and plan to change and see if that helps, I've also read that the B-2 piston on these models is a problem. Any suggestions?

thanks.