It's a common issue. Yup, washers as shims work well to raise. Lowering can be a bit of a deal. I've heard tell of people removing wood from the MPF to lower. However, before going there, I would look very closely at the brackets themselves OR you are missing a roller.
If you're missing one of the rollers, it's bad. You need the side rollers AND the roller on the cam... it's not common but I've seen this before.
The PF is flat... the MPF is flat. Yet the MPF rides the bracket wonky... so, my $1 on the table is that the bracket is not square. It can happen with a ball getting stuck between the PF and MPF and the motor keeps cranking (lots of torque).
Pull the MPF assembly again, separate the brackets from the MPF wood and even pull the cam off.... just get the brackets and run this up and down by hand over and over again. It needs to mover really smoothly without wobble.... that's the goal.
REMOVE THE ADJUSTMENT BAR ACROSS THE BACK OF THE BRACKETS (or at least loosen).
Use wood clamps to put light pressure to press in/pull apart the brackets to alter shape. You may need to over tweak a bit to have it spring back to the desired shape.
I found a document sent to owner/operators back in the 90's regarding a "canting procedure". I posted to IMDB a few years back.