Your cooling system has to be checked and / or one or more circuits need to be cleaned.
Explanation:
If the temperature difference exceeds a certain value, mechanical mis-alignments may occur which can affect the welding geometry. Obviously it is not the same when a welder is cold (start of production) and hot (after up to1h of production).
NOTE: With Can Man TempGUARD™ system, this can not happen anymore.