I had this same exact problem. Drove me nuts, and I live in AZ, where it gets 110+ in the summer. At stoplights and drivethrus, my temp would creep up to 240! Here's what I did:

1.Installed B-Cool Radiator and overflow can.
2. Brand New Stock Water Pump, not high flow or anything like that.
3. Miloden 180 thermostat
4. Flexalite 210/220 dual puller fan (this was after trying highflow flex fan, HD Fan Clutch, New Viscous fan, etc etc etc - none of it helped)
5. Flushed and drained system til it ran clear, about 6 or 7 times...


Hope this helps, it worked for me. I didn't go through all the posts, but what do you have for an overflow can? This is important because when the radiator gets hot/pressurized, the excess fluid goes into the overflow, then is pulled back in when needed. I can't stress enough, check the small things. Make sure there's no air pockets or something like that.

Good luck, I feel your pain.