Speed Problem with Merrill turntable

I have a Merrill turntable originally purchased in 1985 that I had upgraded about one year ago, with a new motor, power supply, tuning weight, clamping ring and springs. A complete makeover. I am finding that the table slows down after about 1 hour of use, and sometimes almost crawls after a few hours of use. It was much worse during the warmer weather than now, in the winter. I believe the motor might be overheating and slowing down.

I cannot get the upgrader to respond to my emails or phone messages. He sent a replacement motor about 5 weeks ago, but the problem persists. Has anyone with a recently modified Merrill experienced a similar problem, and if so, how were you able to correct it. Thanks.