Juddering and revving high

My Rover 25 judders constantly whether I’m sat in traffic or going along at speed. It also revs very high and it has cut out before, although it hasn’t done this for a while. It has been to 5 garages, all of which put the car through a diagnostics test. 3 times it didn’t show any faults but then the next time it showed about 12 faults, although the mechanic said it couldn’t be all of them. He thought there must be a fault in the wiring loom, so he checked the earthing and it was all ok. He ended up changing the air map sensor switch which strangely cleared all the faults. The car was fine for 2 weeks but now it is back to how it was, juddering constantly.
I just don’t know what to do now, it is costing a lot of money keep taking it to a garage. Any ideas?