My shop is about the same size, etc. I have a 60 amp drop and about 150' of
cable run. The wires are individual #6 AL.

I am not surprised at all that you are having problems. #6al is only good for
50a at it's 75c rating. The base 60c rating is 40a. Add the 150' length and you
can expect the line sag.
At 60a you will be dropping 14.5 volts