Karl,
What kind of pressures are you using out of your compressor? I also have an electric heat pump and air conditioner along with baseboard heat and have no problems. And if I'm not mistaken your area of the country is not much drier than ours. Like you, I go about 50/50 or so. How old is your paint, and have you mixed it thoroughly? My initial suggestion would be
1. If possible, lower your spray pressures slightly.
2. Try thinning a batch 60/40, 70/30, and 80/20 and spray at a consistent pressure and distance.
3. Try using a standard mineral spirits thinner (like what's sold at a hardware store). The model master airbrush thinner is "hotter" than regular mineral spirits, and may be flashing off on you at an accelerated rate.
4. Most importantly, let us know how it goes...
Gip Winecoff