well it might be if it didn't have one before you put on in then i would think its being over heating before now. check the oil on the dip stick see if its creamy looking if so head gasket needs replacing if its black the oil then it would seem ok from a gasket point of view if I had this problem I would get radiator flush remove the thermostat then put the flush in reconnect the pipes etc. run it , it could also be heater matrix is blocked hence over heating due to water circulation not getting passed you can by-pass the matrix by simply take of both pipes join then together so that you bypass the matrix .