I don't know much about this subject, but in the CPA class, they said that oil breather lines are at risk of freezing closed - and that a well know fix is to cut a notch in the hose a few inches from the end (such that the notch is just inside the cowling. The notch serves as a backup vent.
As I recall, the notch looks as though it was created by simply sawing about 1/4 of the way through the rubber tube. I asked my mechanic about this - and he acted as though it was a well known issue and the solution was common practice. See if your mechanic knows anything this - or you might drop CPA a line.
A different approach would be to "debug" the M20 oil air separator. Since those work based on gravity - the path from the separator back to the engine must be as "downhill" as possible. Oil is returned to the engine a drop at a time, so there must be a clean path for this to flow (no flat spots and minimum turns and distance). If you're returning the oil into a valve cover, the line must enter the valve cover above the oil level inside the cover (something like 2" up from the bottom of the cover).
If the M20 was misbehaving, I think you'd simply see excess oil on the belly - so this is probably not the root cause of your problem.
Bill Sandman at M20 is a very nice and helpful guy. You might consider sending this question to him. In the past, I sent him a digital picture of an M20 installtion that was causing trouble, and he very quickly had a solution. Maybe he'll have some ideas on this.
http://www.m-20turbos.com/