Ford Fiesta intermittent overheating

Hi, Had this car about 5 years without issues. Oct 08 the the temp warning light was coming on. Discovered the oil and water were mixing.They removed oil cooler and sent for pressure testing, was OK, they drained the coolant, flushed out the pipes and radiator as best they could, by passed oil cooler refilled cooler and bled it. Think they discovered it has a focus engine if that makes a difference. Temp light still kept coming on. They cleaned it through again. Still kept coming on. It completely overheated in March this year.Unable to confirm head gasket and suspected partially blocked radiator. New rad fitted, did something with the rad return pipes,bled cooling system and fitted new exp cap. Fan was cuttuing in and out ok. This light keeps coming on and I have to drive with heater on and fingers crossed. This can come on after 7 miles on a motorway, or after 100 miles, or not at all! It's driving me nuts and cost is getting silly now. Any ideas? Thanks in advance.