It’s used in a bio chem research lab. Its works to start with then slows dramatically; it was the users who sent it to me with this fault. They also said that it speeds up sometimes, which I have not witnessed myself. Because of this I was wandering if it was the control circuitry at fault not the motor.

H'mmm. It sure sounds like a "bad connection" situation somewhere. Heat related (dry soldered joint) perhaps. Either that or a dried-out capacitor.