[Engineer Ed]

Jim,
My thoughts are that the temp issue could be caused by a lean condition.
a rich engine will have less HP but the cooling effect of the gas is the trade-off. A lean engine maes more power but will run hotter.
Could it be that at lower speeds ( minimal throttle) your F/A ratio is less that at higher RPM's?
Try running at a slow speed but high RPM and see what effect that has.

Jay, the cap pressure is up to you.

Water under pressure boils at a higher temperature.
At atmospheric pressure water boils at 100 deg C.
At 11 PSI, water will boil at 121 deg C.
At 15 PSI, water boils at 125 deg. C....
You can run as much pressure as your system can handle.
The benefit of a higher pressure is that water must get hotter to boil, The downside is the stress on your hoses and gasets with a higher pressure.

Ed