1999 VW Passat V6 2.8 overheating

I have a 1999 VW Passat 2.8 V6 overheating on city streets only. No check engine light and has 122k miles. On highway it runs great even with AC on, electric fan is working, clean radiator (not clogged),clean fluids (no cross contamination such as oil coolant mix) The fan clutch seems to be good, clean exhaust while running. Its has a hose on the coolant tank from the aux. electric water I never see it running, but VW dealer said that would not cause it to over heat. I replaced water pump and timing belt 60K ago. I have owned this VW since new and maintained perfect with VW OEM parts. Also I live in TX where summers are extremely hot! What else would cause this symptom?? Second question would a thermostat stuck open cause this problem as well I have not check it yet?

Try replacing your coolant temperature sensor. Get the one from the dealer. If that do not fix it then yes, I would check T-Stat. Franco

Probably need a new coolant temp sensor if it hasnt been replaced. They corrode and short out.