hi, I have a 91 tt from japan. Car runs pretty good but when I get on a red light idle drops to adout 800 and run smooth. But maybe about 15 sec later, idle would drop to 650-700 and car will run rought. not misfiring!! no boost leaks, new plugs, new fuel filter, new air filter cleaned the IAC valve assy. just can't find the trouble...anyone had a similar trouble??