Not unusual to run a synchronous motor on a lower voltage in order to reduce vibration, but 76V on a 110V motor does seem rather low. Can you identify what type of motor this is? (I'm suspecting a Premotec or similar..) I'd figure 90 - 100V would be more reasonable.
I experimented extensively with lower voltages and noisy synchronous motors and found that some motors ran quite well at 70% or so of their rated voltage, but torque was greatly reduced obviously.
Sorry to hear that rather than provide the information to fix yours they just want to sell you one of their upgrades. Frankly I'm not too surprised..

I had a slightly used PT turntable (model now forgotten, and I've tried really hard to forget I ever owned it) in the early 1990s and the ruby in the inverted bearing shattered. Before that I had terrible problems with the dc motor based drive, it had the poorest speed stability of any table I have ever owned. (The motor was widely used in cheap Japanese belt driven tables of the time which had no speed stability issues, I had two controller boards and motors, both just as bad, clearly a design issue.) A shame because it clearly incorporated some interesting design innovations - inverted bearing, fairly heavy acrylic platter, and a honeycomb sub-chassis none of which were very common at the time. Because of a dispute between them and their distributor all American owners were left high and dry with no support from PT. I was not able to get parts to repair it and ended up selling it at a big loss to a fellow in the UK who wanted one and planned to have it upgraded.
I'd inspect your bearing, and also determine whether or not the motor needs some lubrication.
Can you provide any more detail about the supply - is it just a transformer and some resistors and caps or is an actual source that generates the driving voltages required? Could need a recap?
Can you take some pictures of the controller?