OVERHEATING 730i

my 1989 730i overheats when the car is idling.no probs when moving soon as i hit traffic it will creep up to the red on the temp gauge.The new Viscous fan seems to work fine.I have also fitted a new water pump but to no avail.Over the years i have used some Rad-Weld,although i have flushed the Rad&Engine could there still be a residue left in the system?Any help would be appreciated. Cheers