D-LYLES

I`m having a problem holding my coolant temp in the area it needs to be in to let the overdrive in the A/T work properly. Just finished a head gasket job including waterpump , timing belt , 4 different thermostats, flushing coolant system and burping coolant system numerous times. Operating temp at idle takes 15-20 minutes. Take off down the road Trans will shift properly then you can watch the temp gauge slowly go back to cold and thus trans will go back to 3rd gear because coolant has dropped to below 160F. And yes , i have tried the TOYOTA brand thermostat @ 180F , a NAPA dual stage, a 192F fail safe from AutoZone and a cheap aftermarket. Nothing seems to help including blocking 1/3 of the Aftermarket radiator. Answers will be appreciated.