Tiny Air Bubbles in polyurethane HELP

Ok, SO I tried to warm the room up, and even warm the urethane with warn tap water. I still get the tiny bubbles. Its almost foamy at very first when its going on. I AM NOT shaking the can, I only stir it. I asled the wood guy at Lowes and he said its my brush. He tried to sell me a china white brsitle brush WHICH I ALREADY HAVE!! I am on my 4th coat and no luck with the bubbles. They are small and few but they are so noticeable with the semigloss. I bought some of the wipe on polyurethane stuff by minwax, It seems thinner and I hope it works.

ANY Ideas as to why I am having this trouble??

1. I am sanding to fine finish with 220.
2. I use tack cloth to remove all dust.
3. I use sweeping brush strokes in one direction.
4. I let dry in room at 80 degrees for 24 hours.
5. I come back to find a "pimply" finish

HELP

 

My shop tends to be on the cool side and what I have done is to warm the finish by placing the can in a container of warm tap water.
If I raise the temp of the finish to about 90 deg F the bubbles go away..

 

Ok. I have the window unit on in that shop (its probably 60 degrees in there )
I thought it would help it dry because its so humid in texas. I will warm the can by soaking it in hot water and give my third coat a try. I will also turn ac off to warm it up in there. Meanwhile I will sand the few bubbles out of 2nd coat.

Thanks for the tip!!!