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Has there been any warning lights come on the dash since the fualt?
What tests/checks have been carried out or items replaced so far upto now?
Sounds like you may have a sensor fault or possibilty of over fuelling - one sensor you may want to try and replace is the ECT sensor (engine coolant temperature) this doesnt generally control the cooling fan but the ecu uses this to sense how hot or cold the engine is and adjusts fuel.air mix to suit so that maybe one option to try at the same time maybe a general clean up of the throttle housing/IAC valve and maf sensor and inspection of all vaccum/intake hoses for splits or leaks..- also check the CPS sensor (crank position sensor) as they to can cause similiar faults..ideally as you'll know any checks would need to made when the actual fault occurs to know for sure what needs inspected....