The original electronics used a feedback system to check platter speed. Sounds like something with the feedback or servo is broken. Probably your motor is OK, but if SOTA recommends a new motor to go with the Phoenix electronics, I cannot over-state the benefits to be derived from letting them do it. You'll be amazed.
Funny story: about 25-30 years ago, when SOTA first came upon the scene, they offered a turntable as a kit. A friend of mine who was a very capable engineer decided to build the kit. When he got the turntable completed, I was invited for its maiden voyage. The object was just to see if it worked. We sat across from each other at a table upon which the SOTA was supported. He pressed the start button, and at first we were both pleased to see the platter start to rotate and quickly reach what was apparently the correct 33 rpm. However, it didn't stop there; it continued to accelerate, faster and faster until I thought it was starting to rise up on the spindle, and I feared it would literally take off and hit one of us. I thought seriously about ducking under the table for protection. Fortunately, at that point he cut the power, and we both started laughing. He had miswired that servo feedback loop.
Funny story: about 25-30 years ago, when SOTA first came upon the scene, they offered a turntable as a kit. A friend of mine who was a very capable engineer decided to build the kit. When he got the turntable completed, I was invited for its maiden voyage. The object was just to see if it worked. We sat across from each other at a table upon which the SOTA was supported. He pressed the start button, and at first we were both pleased to see the platter start to rotate and quickly reach what was apparently the correct 33 rpm. However, it didn't stop there; it continued to accelerate, faster and faster until I thought it was starting to rise up on the spindle, and I feared it would literally take off and hit one of us. I thought seriously about ducking under the table for protection. Fortunately, at that point he cut the power, and we both started laughing. He had miswired that servo feedback loop.