Bingowings said:

I know Michael Pennington can act but he is comically awful in ROTJ (he looked like he was going to pee his pants all the time Vader was onscreen).

Julian Glover who is my favorite Who villain...probably...and perfectly fine in ESB is similarly awful in Indiana Jones and the Last Crusade but Denholm Elliott and John Rhys-Davies are pretty awful in that film too and there are no doubts about their talent.

One could argue the stress of the job was getting to him. After all, high ranking Imperials don't get fired, they get choked. ;)