I've got a 2000, 24-valve, 150k that randomly overheats. I have seen absolutely no correlation between outside temperature, engine load, rpm, or speed. Sometimes it runs hot, other times it runs fine. Replaced water pump and thermostat. Cooling system holds pressure. Stumped, I took it to a mechanic who claimed to detect hydrocarbons in the coolant. He says this indicates a blown head gasket. Would a cooling system still hold pressure with a blown head gasket? Thanks!