I had the same problem with my 283. In this case it ended up to an air pocket that kept the water from moving. I came to that conclusion because of the difference in temperatures between the upper and lower radiator hoses. No matter how I tried to relieve the air from the system, nothing seemed to help. Someone on this forum suggested the old trick of drilling a small hole in the thermostat to bleed the air. Worked great, temperature is rock steady.