It could also be the gauge. Mine is doing just the opposite of yours. I finally figured out my gauge is off by 150 psi. It drops some after I start working, but when I finish and turn my pump off the gauge reads 150 psi rather than 0. I already have a new gauge purchased. I see no way to correct the bad reading on the old gauge.
It might be good to install a pressure relief valve, actually it would be smart for every truckmount to have one just in case of an issue like this. The valves are adjustable to what ever psi you want to set it at. I’d set it just under the max rating of your heat exchanger.Actually Todd, I'm not sure if it stabilizes, because I don't let it get that hi before bleeding it off.