1999 Volkswagen Polo car keeps reving itself

the throttle housings on vw do have issues but if you clean or replace them your engine control unit needs to be reset to learn its new settings maybe the mechanic didnt carry out this criticak procedure. if he has maybe the fault was something else all along such as poor contacts in multiplug or damaged wiring. you need to get the garage to check the work they've charged you for and offer you an explaination of what they think is wrong. if no joy seek a refund - they haven't fixed the fault simply mis-diagnosed the fault

My polo is in the garage relating to the same thing. Revving itself and eventually overheating. Solution 1 was to replace the temperature gauge/sensor - this didn't work. Solution 2: was to replace the water pump. This isn't the problem, it's in good working order. Final solution is that there is a cylinder leak and a new head gasket is needed. It's having a cylinder test later which will pinpoint where it's losing water and causing the engine to overheat.
The overheating is the end result of whatever is causing your overrevving and it could very well be either the engine temp sensor or the water pump but in my case it's neither.