Right after the transmission fluid was changed on my 84' 500SEL it started having problems. It would downshift if the throttle is pressed just a little; sometimes while cruising on the freeway a light pressure on the throttle would cause the rpm to rev up like it shifted out of 4th to neutral then back into 4th; and last sometimes it wouldn't upshift from 3rd into 4th and when that happened the throttle felt very stiff. The shop that did the work has removed the pan and inspected the filter and gasket and couldn't find anything wrong. Any suggestions?

Tommy