A couple of things to check.
- Are the spark plugs the right heat range?
- Is the EGR valve working correctly? It helps keep combustion chamber temp low
- Is the spark timing right? Too advanced timing can build temp
- Is the engine running lean? A clogged carb or injector can keep the engine from getting enough fuel
- Is this an oem valve you are putting in? Did you lap it into the seat well?
- Do you think that cylinder is burning oil? If carbon builds up on the valve seat, it can keep the valve from cooling off.
Scot