Sounds like you're checking the right things. I'd suspect a bit lean - however; I wouldn't worry too much about dull orange headers, bright red, not so good.

The infrared should give consistent readings - but have you tried using a different sender and gauge(s)?

You say you changed jets - did you go from stock to a size richer?

You may want to try 12 degrees initial and 36-38 total to see if that makes a difference.

Is the water level staying consistent in the system, i.e., radiator still full?

Finally, what do the plugs look like?

I know I'm asking a lot of questions - but helps to narrow down the parameters..