Pressure problems, exp. tank, and air removal???

I've got a homebrewed heat exchanger type boiler that worked fine for three winters. This winter during a couple power outages I experienced what I would call mild overheat situations. The pressure relief valve worked as it should. Occasionally over the past couple months I've been experiencing some higher than normal pressures. Yesterday I replaced a bad expansion tank and thought that would fix the problem but it didn't. The expansion tank and auto fill valve are quite a safe distance from the boiler so I don't think they were exposed to extreme heat or pressure. When my floor loops are off temp will build to about 150 deg. and build to 35 psi. When I turn on the floor loops and draw down to 135 deg the pressure drops to normal. Normal pressure is when the needle on the gauge is barely above zero. At least thats where it's been the past few years. Normally the system operates around 130 degrees.

Here's some more information. Total volume of the system is slightly less than 20 gal. and expansion tank is amtrol model 15. There is a float style air removal vent at the high point of the system. It's mounted on the vertical leg of a tee where the main loop makes a down turn and elevated about 6". I've always assumed air would rise and separate at that downward turn. There are no air scoops in the system but the radiant loops have an air vent on each manifold. It's hard for me to believe the water at these temp ranges 125-150deg F can account for the pressure rise to 35psi. I'm leaning toward air in the system and maybe some steam flashing in the system? Do I need an air scoop added to the system?

Thanks for your input guys, this has been a real head scratcher. Tank says it's precharged to 12 psi and relief is pressure only. My temp/pressure gauge is inside right after the sidearm and usually ranges from 125-140 degrees F with almost no pressure reading. The aquastat shuts off the air to the boiler at 130 and the temp will sometimes rise beyond that setting after burning. Earlier today with all heat loads off except sidearm the temp rose to 150 degrees and the pressure went to 35psi., when the temp falls to about 130 the pressure just goes away. I assume the pressure gauge is extremely unaccurate at the 12psi. The boiler just completed a burn cycle and at just over 130 the temp is beginning to rise but is under 20psi with all heat loads off except sidearm. Adding another tank wouldn't be much trouble, maybe I've just been lucky these past few years and living on the edge. This thing has been running winter and summer for four years and this new pressure thing is driving me batty. Sorry about no diagrams etc. I'm pretty computer illiterate no scanner or drawing abilities. Thanks again for any and all input.

Well today I tried adding the old expansion tank with a bad bladder and no precharge to the system. So there's a new (I hope it's good) model 15 expansion tank plus a model 15 expansion tank with a bad bladder. If it made any difference it was minimal. Looks like my next step is a new pres/temp gauge.

Well after all this nervous gauge watching these past few weeks, a new gauge and the pressure indicator is resting on the zero peg. That's where it's been the past few years and am I ever relieved to see things back to normal. The lady of the house just doesn't understand why I get so nervous about such things. Warm house, hot shower what's it matter to her.