Ok, I've been having some TPS issues on my 156, it's been cleaned many times by myself and a professional Alfitsi. It kept coming back, so getting annoyed of my car pulling like a none turbo'd diesel 1.5, I decided to have a proper look at the TPS.
Got the old multimeter out and started to measure the resistance, seemed ok, but as the reaction time on the display has the refresh rate of a snail! I decided AlfaDiag would give me a better result, so £42 later I downloaded AlfaDiag and gave the TPS the AlfaDiag check over. Sure enough through the 'sliding' range there were irregularities. Gave the cable a good wobble to see if that was being an issue, but it checked ok.
I removed the accelerator pedel to clean the TPS, but in the process decided to remove the 'white' slider (carefully as to not break the rotational connector!), and on doing this I spotted that the contact brushes were not sticking out enough. So some gental manipulation, and some switch lube later, I have a fully operational TPS...............winner.