My air problems continued until today when I realized that I needed an automatic air vent placed at the highest point in the system!  Air rises through the pipes and gets trapped, when the pumps kick on the air pockets that have formed get pushed around and get trapped in the pump causing the cavitation that I tried to fix a few days ago. (Didn't solve the problem - see below!)

So I went to the hardware store and bought an automatic air vent.  This doodad is the type of thing that you see on the top of classic radiators.

Here it is installed at the feed end of my 2nd floor baseboard radiator, I replaced the elbow that feeds the radiator from below with a tee and reducers to the bushing for the vent.

Later:  Well that wasn't it either!  My pumps still cavitate!  However I highly recommend the practice of installing air vents at the highest point in each separate loop.

April 2, 2005 problem solved.  I needed a bigger expansion tank!

Another part of my pump cavitation issue is to ensure that when I pressurize my system that the water is at or below 70F.  If I fill the system while the water is say at 100F then when the water cools it contracts bringing the  static pressure down below 10psi.  That's when the pumps start to cavitate.  The utility pump that I use to fill and pressurize my system can only provide 15psi.  - compared to house pressure which is about 40psi, but requires that I run a 70 ft. garden hose to my workshop which does not have running water!  That's a huge pain when there's snow on the ground and it's 10F outside!  It  turns out that 15psi is the bare minimum static pressure that will cause the pump and the propane heater to operate. 

At the end of a good solar day, my tank pressure rises to 100psi and the stored water is between 130 and 160F.   April 2, 2005 update:  After installing the larger expansion tank I no longer see pressure variations!  Yaay!