Originally Posted by HammerZ71

Yeah, you could try the cooler thermostat, it won't hurt anything, but if you're going up to the 3/4 mark, the thermostat opening 15* sooner is probably just gonna buy you a minute or two until the temps reach where they are going to anyway.

If you can't find anything wrong, and I don't know if the radiator being slightly smaller is enough to cause the issue, there is a product made by Royal Purple, it's called Liquid Ice. It basically has additives in it that will cool better than just a 50/50 coolant mix alone as well as some that help break down any rust or other contaminants. I've used it in the past with good results, it's not going to make a night/day difference but I've found it will keep the engine temps down an additional 10* or so.
Comes in a 16 oz. bottle and you should take 16 oz. directly from the radiator and poor this in. It's not terribly expensive, over here at least. I think retail is around $14.95 a bottle and the last one I bought was about $12 at the auto parts store...