If you are certain you have eliminated leaking exhausts, then it sounds like a headgasket issue. unless you left water in the motor and it froze over the winter possibly cracking the heads and or the cylinder bores.

It seems really odd that just the front two are having issues.

could you post a video link of it happening from smooth startup to when it starts missing? You tube or something...

I'm in California. Do you really think that can happen?

Burnt valve that acts up when getting warm??

I have good compression. Would I still have good compression with a burnt valve?

http://www.earlycuda.org/tech/vacuum2.htm Head gasket

I dont know does it get be freezing, where you are at in cali? Water turning into ice will crack metal easier then you snapping a toothpick.

If you think its a water issue, just remove the water from the equation as I have recommended prior. You wont overheat it idleing for 3 mins.

Well I figured it out!! STUCK VALVE. Did a leak down test and found air coming out of the #2 exhaust port. The reversion was from the open valve that was sucking water and shut down my spark plug. So I pulled the head sprayed WD40 on it and it went right back in but seem a little tight. No salt water reversion it's from the hose.

Is this happening because it hasn't been started for four months or should I look for something else?

What should I do to fix this?

Might want to take the head to a machine shop and have it checked, guide clearence, bent valve, worn out springs, just a thought.