How hot is it getting?

Like Zef said, did you bleed the coolant system to get all the air out of it? If you have an air bubble in the thermostat housing, it will not operate properly.

Have you tried replacing the thermostat? for what they cost you may want to look into this.

OR
does it only heat up in traffic or does it get hot on the highway also.

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GreenBastardPerformance2000 GTP Coupe (the green bastard)
XP, Pacesetters, LS1 TB & MAF,MSD System, P&P LIM, P&P GenV w/ 3.2, 3.0, 2.8, Devil'sOwn Injected 100% Methanol,PowerTuned, TripleEdge Trans w/3.29's
12.92 @ 105mph