yellow patches in poly

I have never had this problem before. I am finishing a solid bench made with pine bead board applied to an MDF base. I had stained it to my satisfaction and so started to put on the protective satin poly finish, as I had done on many other projects. But as it dried, patches appeared in several different places; they looked like flaky, yellowish ecxema patches.

Well, thinking it was because I had used an old can of poly with an older brush, I did some repair work with sanding and oil paints, got out my new can of poly (gently and thoroughly stirred) and a different brush and went to work. Same result! The patches are now in other spots! This is the same brand of poly I have used for years without a problem, same technique, a tried and true brush (well cleaned), same garage. I have no idea what is going on. Any thoughts about what is causing this? And how to fix it?

Im guessing there's either something in the wood causing the reaction, or are you using a type of grain sealer thats affecting the top coat?

I used the same progression of sealer, stain, and poly that I had used a dozen times before without a problem. I talked to somebody at my local homestore and she thinks it was the space heater in the garage cycling on and off. I don't remember if I finished with the space heater on before, maybe I didn't. I certainly won't do it again.

I could understand the heat affecting it but only if the variance of temperature was above or below the temperature rating of the paint used...

Well, since the space heater was about 3 feet away from the surface....., We all do stupid things sometimes, don't we?