Could be block rust scale, if the block is not properly cleaned in the water passages there can be a scale of rust, that scale of rust will cause a thermal barrier, and prevent proper cooling. You can use a cleaner call Thermocure to clean the passages out, the process takes about a week, you add water and the Thermocure to the water and use as normal, apparently the longer you leave it in there cleaner the iron block with get so might want to leave it in couple weeks, then wash out the systems several times make sure you use the coolant drain point lowest on the block also. You can re-use the mixture also so get a 5 gallon bucket to keep it and use again.