My 99 Seville STS is overhearing bad!!!?

I own a 99 Seville STS and about 4 months ago my water pump went on me. Since then I had the pump, pulley beqrings and belt replaced along with the thermostat. My car overheats like crazy as soon as I'm at a light waiting, before it turns green the temp needle goes to red unless I put te gear in neutral and rev the engine to about 3500. I have to drive through the city in 1st or second gear depending on traffic. I have no gunk or white smoke out my tailpipe and no coolant in my oil, so I'm not sure it's a head gasket problem. Could this be a faulty thermostat? Blockage somewhere ? All rad fans are working . Im not sure what my issue could be? I added a head gasket leak additive to stop leaks and nothing is successful. I'm not sure I even have a leak? Any thoughts? A pressure test was done but was not done properly I think because the cap kept popping off , which is what I noticed from watching the mechanic. Nonetheless less I have no idea what could be the issue? Bear with me if I sound like I'm all over the place , I just hope my problem is fixable. Cheers Ryan !