I posted a while back about my transmission being plagued with converter/clutch shake....well, then I had the fluid drained (absolutely disgusting thanks to the old owner) and a new filter installed.....shaking goes away for about 4,000 miles, until about 300 miles ago where it shows up again on a drive back from South Carolina...now it's not as bad as before, but it's still there. Question is, can there be something else causing this, or is the transmission's life been prematurely cut short thanks to shoddy maintanence by the previous owner??

 

I'd try changing the fluid again. If it was really nasty before then the filter could be clogged again. A simple pan drop and drain never gets out all the fluid anyway. It couldn't hurt. As far as damage goes, it's hard to say, but dirty fluid is never good. Best of luck and hope it helps.