I have a C3500 I rebuilt the motor and replaced radeator waterpump etc..
Its all new. I put a 180 thermostate in it and when driving it runs 190 to 210. when I get stuck in stop and go traffic the temp will go to 24o +. fan clutch is good and radaitor cap is new.

 

Sounds to me it's one of three things. EIther your radiator isn't large enough for your application, your waterpump isn't working quite right, or your fan isn't enough. I'd do two things.

1. Dump the clutch fan for an electric fan.
2. Check out Jet performance systems, they have a fan switch (for electric fans) that will turn your fan on at about 190* or so. Most stock fans don't turn on (even the clutch fans) until 220* or so.

If this doesn't work, check out "Becool.com". They've got some great radiators for all applications and should work for you. Best of luck to you!