Help - Rover 25 unfixable fault??

My W reg car has had an intermittent fault for months - engine catches and jumps, and revs go wildly up and down, often settling around 2000 when clutch in. Wonderful garage been trying for ages, replaced every chip in ecu and changed rev control valve, changed ignition coils, ht leads and plugs. Diagnostic computer brings up inaccurate fault codes or the car won't 'talk' to it at all - they're losing money trying to repair it as they can't charge me when it's not fixed. That said it's cost me nearly £800 so far, and I can't even trade it in until it's sorted. Drive fine after fixing for a day-a week, then all starts again. Any ideas???