i just purchased a 1995 pontiac trans sport minivan.i have already had it run through a safety inspection and a e-test ,which it passed with no problem.however,when the van is first started, and while it runs until it warms up, it keeps wanting to stall out.once it does warm up, it runs ok .it takes approx. 20 mins. for the van to get working properly. and it is all the time. hot or cold now plz help me