A little advice needed..

Hello, any advice on this would be very appreciated.
My grandmother has a 1994 998cc model,with only 25k on the clock.
It's developed, what appears to be a common fault,which is violent "grass hopping " which kicks in only once the engine has reached normal running temp. So I bought a tested second hand throttle body, swapped them over ,ran it on idle, went for a test drive for 10 mins,it was running perfectly & then suddenly the problem returned.! The car is about a hours drive from me,so not very accessible.
I'm unfamiliar with these engines, and would be eternally great full for a bit of advice,from someone with knowledge. Her mechanic took the mick & didn't find the problem ,so I said I'd have a look.
Jack.

the lambda readings when the fault occurs will give a good indication of whats at fault jack, just a volt meter tapped into the middle wire :)

Terrific, I'll do the test when I'm next over there & see what the readings are.
Thanks again. :)
Jack.