If you want to start small why don't you buy that stock fan and shroud for $30?? Honestly the POS that's on their now is probably the problem. You say it starts to overheat when it's sitting still in traffic or at the gas station? Doesn't take a rocket scientist to figure out that it sounds like not enough air being pulled through the rad. You start driving and it doesn't overheat right??

Even if it's not the fan at least when you do figure out the problem you'll have good cooling...

Thermostat would be my next guess after fan. Good luck.