Disco overheating

I have recently become a land rover owner of a 1998 Discovery 3.9 V8i Auto it is an absolute pleasure when I drove it, - 265000kms and then it started going wrong, - and the vehicle seems to get hot periodically, I have replaced the Viscous coupling, had the radiator redone, fitted new water pump, new hoses, new thermostat. had the cyl heads redone and replaced all head bolts, new top set, timing is correct, CO was set, new expansion bottle cap, just about everything I could think of.