Megane 1.4 problem with revving

[Bradwood]

Hi Recently bought a megane 1.4 2004, when first got car home noticed that the revving was idling at 2000rpm, had a diagnostics done and found it was the altananator, replaced altanator with brand new one now the car is fine for a couple of days then has a mad one and revs up and down on its own, also has a nasty shudder, sometimes it seems it under revs now when it's cold? Had another diagnostics done and no fault, cleaned throttle body also the engine fan is now on permanent and won't go off. Any advice anyone. Cheers