in my 351 block i had an oem 195 deg thermostat with holes drilled top and bottom and the temp would fluctuate too much and reach too high a temp on initial startup before it would settle back. switched to a stewart 180 deg high flow thermostat from speedway with holes drilled and at 70 deg. light cruising temp would be 150, get on it and temp would rise to 180 nicely. might be something to check out.
http://www.stewartcomponents.com/tec...ech_Tips_3.htm