Panzer, I just read your other thread, with several comments by my Dallas co-conspirator - Hit Man.
Yeah, it's gotta be the monovalve, and the plugged screen sure wouldn't help.
Sometimes you can't really see a defect in the valve or diaphragm, but it just doesn't open properly. It will really thump when it's popping open and closed properly.
There is also a small auxillary water pump next to the monovalve, that actually will turn on to pump more hot water during prolonged idling in very cold temps. I unplugged mine years ago when it started to mis-behave, and have not noticed any difference here in Dallas. I doubt that is your issue.
If you are getting air from weird vents, it could be your vacuum dashpots leaking.
I'd try the monovalve kit first before messing with those. Let us know how it works out - I've replaced all that stuff over the last 18 years, so we can walk you thru it if the valve doesn't fix you up.

DG