Bob,
My guess is that since it is only when turning Left, the problem is with the "bow-tie" or something mechanical in the break that is causing a lot of drag on the motor when turning left. Try measuring the voltage between lug 6 & 4 turning left and compare to the voltage between 6 and 5 turning right. I suspect the load or drag is drawing down the secondary side of the AC transformer, affecting both the feed back circuits and the drive motor.
Easier said than done, since as you state the issue is intermittent and the switching action clears the problem.
When you say the voltage pots do not change anything, I suppose you mean adjusting them does not fix the problem. That is, it is not a dirty pot issue.
BOB KO6TZ
Apparently the internal 6v supply is dropping since both meters glitch down and an externmal voltmeter on the rotator feedback terminals also show the same10% drop in the 6v line. (though it appears to be a constant 5.5 volts not 6) And then glitches down from the 5.5.
Also noticed the Adjust voltage pots on the back do not change anything.