1998 Deville Concourse
New Radiator, new Tstat, flushed out system. Car maintains regular temp on idle. Once I take it out on the street it starts to overheat. My last resort is to check the "vapor line" or how it may be refered to as. Read 60 pages worth of post trying to figure something out. Also spoke with previous owner and claims he put some kind of radiator stop leak sh!t in the system. Please advise.