Vauxhall Elite 3L V6 overheating?

Hello. Bought my X reg elite for the heated seats. Drove a couple of hundred miles ok then the temp gauge went into red. RAC said nothing showing on computer but replace thermostat and sender unit. Had this done (pain of a job) but did same again. RAC said fan only kicking in intermittently. Its only doing it while engine is idling or low revs and if you keep driving the gauge returns to normal. Its had a new water pump. Thinking of having water pump replaced and heater bypass valve (suggested in another forum) done while the engine is in bits but worried. Am I damaging it while driving. Will this latest repair solve the problem? Any ideas welcome. Definately not a head gasket problem. Garage stumped. If these repairs don't sort it he thinks it may be electrical.