aliS

Have you ruled that the issue could be with the coolant side rather than engine? Is there any noticeable oil leaks or consumption? Is there a change in appearance to the engine oil on dip stick or cap? If not maybe your coolant system is just gunked up and under increased load insufficient radiator coverage is available to maintain the desired temperature? A coolant flush might be a good cheap place to start using a suitable cleaning product in the mix before draining.