The 2 valves may be fighting each other as the pressure changes and vacuums are created when the system cools at night.
you might try only 1 valve and see what happens
Thanks WindriderNM, but I don't think that's an option. The two checkvalves isolate the panels, expansion tank, and pressure relief valve from the pump and heat exchanger below them, when the pump is off. If I open the valve on the hot side, it will make the thermal syphoning worse! If I open the valve on the cold side, the expansion tank, and relief valve may not fully protect the system. For example, if there is a flash to steam event, with no checkvalve on the cold side, it would allow the glycol to be forced down passed the expansion tank and through the pump in the opposite direction of normal flow. This could damage the pump over time.
I think you're right about a vacuum forming on the panel side, but I think if I add another 5psi. or so to the system, it may cure the problem
Thanks again for your input WindriderNM