I figured it out. In the process of replacing the thermostat with a 160 degree unit, draining and refilling the cooling system, I discovered a large air bubble in the cooling system. I put more coolant in than what drained out! While I was cycling the engine to 230 degrees, then slowly opening the coolant fill cap, I noticed bubles working their way up to the overflow canister. After the third cycle of heating up, then cooling down, a large woosh sound and the coolant level dropped. Now the engine finds a happy medium around 198 degrees and varies only a couple of degrees, even in 90+ degree wheather! Dennis