is this something that has always been happening, or did it start recently?
Is it a V6 or I4?
are the heat shields in place?

an overly lean mixture, or an overly advanced timing, can lead to excessive combustion heat, if the over heating of the manifold that you are experiencing has only just started doing this

a air leak in the intake tract could lead to an overly lean mixture

I had mine ceramic coated (inside and out), which helps reduce under hood heat a little bit.

 

I suspect it's always been this way. Yes it has heatsheilds as far as I know. Tell me more about this ceramic

 

So ovey advanced timing means the belt is about to go?? Last time it was changed I seen it was marked at 169,000 or so miles it now has 320!

 

"So over advanced timing means the belt is about to go?" - no, there is no relation. Ignition timing is controlled by the distributor. It is checked using a timing light.

 

Ceramic coating requires removal of the header, media blast, coat and bake. I've had it done on every car I've rebuilt and like how it turns out. Note that it is just a coating, so the amount of heat reduction is very modest. There are many sources for having it done, Jet Hot is the biggest company, also there are smaller ones who use the same chemicals and have the hot enough oven for it. There is also Cerakote, which racers have used for years, it can be applied cold, and they also have a type that can be baked.