I think new thermostats are not as accurate as old school parts, like everything else these days, just poor quality. Mine does the same thing, 2 new stats and the same scenario. The gauge will rise to 180 - 185, then drop off to 170 and stay there, I block off part of the radiator with cardboard and I can regulate it quite well.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy