There was a thread on the TPS needing adjustment sometimes, on TBI EA's, but yours being MPI, I doubt that's it.
There's a very large chance (being that the fault started around that time) that the water is the cause of the fault... and I know that this doesnt help you but that's the reason why I only ever wash the engine with degreaser cans, and only ever wash the degreaser off with more degreaser. if it needs a wipe out get a sh*tload of rags... water doesnt ever come near my engine bay except rain
I'd say either your dealer made a mistaken adjustment - or your problem is the TPS.
Lets face it - his adjustment did make a difference because it's not hunting anymore (at least thats how i read it), but obviously he didnt outright fix it. Maybe he just picked up the idle so it wouldnt stall and you'd leave him alone?
I think i'd best leave this one for someone who's had the same problem.