I have turned 200 miles on a newly rebuilt 1971 429 CJ engine and have been taking it easy for the first 500 miles until it is properly broken in. Twice now the temperature has spiked to the far right (pegged the hot side) after 20-30 minutes of driving (25-50 mph). Since I have been driving locally I have garaged it so not to do damage. I thought it was an air bubble in the line but have sinced ruled that out. Water pump et al are new. Generally, it registered slightly above midpoint. Anyone have ideas as to the cause? Thanks.