Revs can be erratic when idle

I'm totally stomped. I have an MGF N reg. When accelerating hard so the revs are high and then immediately disengaging the clutch my
engine gets stuck at high revs for 2-4 seconds. Also when starting my car the revs can either go too high or nothing at all forcing me to accelerate to prevent stalling. Any advice would be much appreciated as nobody seems to know the solution. Some people have suggested the ECU. Is this looking to be n expensive job or can there be a short term solution for now?
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