[Shon]

when driving the temp is fine?
Remove T-stat and see if water is flowing and if that solves the issue.

I had a LS1 swap and driving around things were fine but at idle/red light the temps would get up in a hurry. I replaced water pump no change. took out thermostat and all was fine.

I know you said you replaced the T-stat but it could be faulty too....but if you removed it you can tell if the pump is moving the water.