FICHT and ETEC engine run with a computer aka ECU or EMM ... both can be accessed with EV DIAGNOSTIC SOFTWARE.....and you shop should have plugged in to detrmine any faults in the history file.
if changing the TPS did nothing the shop should have found a fix or maybe it is best find another shop !!!
i will post a write up on EV diagnostic....and a book to read on FICHT engines
see a comment from the ETEC user board from years ago....
It sounds like you could have a fuel lift pump diaphragm leaking. If the diaphragm ruptures it can leak fuel into the pulse hose which is connected to the crankcase. When this happens, the engine gets an additional charge of fuel and will idle up. It will go away temporarily when you rev it up because you are clearing the additional fuel out of the crankcase. You would think WOT would be affected but, since the vapor canister holds a fair amount of fuel, the lift pump still has enough efficiency to keep it filled at WOT.