96 chevy Blazer trany burping

I have a 96 chevy Blazer; automatic and 4x4, that is building back pressure in the transmission and burping out the vent next to the dipstick. I’ve talked to a couple mechanics and had them drive it and they say the transmission is running well, they just don’t know why it is building backpressure. Two weeks ago I had the transmission flushed and the problem stopped until yesterday, now it is back at it. I’m ready to take it to a transmission specialist. I live in a rural area so that involves taking at least half a day off work. Does any one know what possible could be wrong and how I can fix this problem?

Vent not working right, can it be unscrewed and cleaned?
Even if it is building pressure it should be venting all the time not every once in awhile.

Oil pump worn, trans overfilled, fluid contaminated with engine coolant. Definately let the trans shop look at it.