Hi, Freon in this case 134A, has a psi/temp. relation. Too much psi in the system will cause a higher temp. When you are run down the road it is getting cold its because your engine is at a higher RPM, this is causing the comp. to pump down the psi on the evaperator coil causing lower temps. When you are at the red light you engine is at a idle, and the comp. is not turning as fast. This is letting the freon psi in the system to rise and temp is not as cool.
So, if you added more and it didn't need it you could have Compounded your problem with another. If you know someone with a set of refrigeration gauges your low side should be about 30psi @ 1800 RPM +-
Also if/do you here Clicking under the hood when driving, as if your Clutch is cycling on and off and your sure you have psi in the system. Possable your low psi cut off is bad. Food for thought. Good luck