I just replaced the thermostat with a brand new dealer-bought one, and the mechanic I dropped it off at said he burped the cooling system for me.

I now see your post where you said that....my mistake.

any leak in the coolant will not only allow coolant out, but air in upon cooling. any air in the system can get re trapped. that will cause all your problems. My ea81 t was experiencing the same problems, unfortunately I never found the leak, but shansf has it now, and I bet its in teh intake manifold

If you need a new radiator, Check with Fitz Auto Wrecking in Woodinville. They sell new Modine radiators and I haven't found anyone who could beat their price. I've had to buy four radiators for different vehicles over the past 10 yrs and they have always been the cheapest. The radiator for my Subaru was $104 two years ago. Went through the same symptoms you have and it was the rad. Good luck.

Did I miss the cliffhanger scene?

 

It sounds like the coolant is "gurgling" through the radiator. Coming through in spurts.

 

Perhaps you are low on fluid the part of the radiator that is clogged is near the bottom, so when you check the fluid it looks full.