Sounds likely to be an expansion tank problem, but you should set up the auto-fill to deliver no more than about 15psi when the system is cold. On most systems you can isolate the auto-fill valve from the potable supply with a separate valve (often a ball valve, sometimes a gate valve), which you SHOULD do. The auto-fill function is best used to simply not over-fill the system when purging air.
With the potable supply to the auto-fill open, you should be able to crack a system drain valve and as the water pressure on the system side falls to the auto-fill's adjusted set point, the auto-fill valve will open up to replace the water that's leaving. So if you hear/feel the auto-fill opening up at some higher pressure than 12-15psi, you should turn it down. When you're in that range, close the drain, then close the isolating valve, just in case the auto-fill has a slow bypass leak (which happens pretty often.) Only if you're starting out at a reasonable pressure will the system behave properly when running.
Then if the system then spikes to 30psi and higher as it's firing you'll know that the expansion tank is either shot, or isn't pre-charged to the correct pressure, or that it's entirely too small for your system's water volume.