Underdrive pulleys and overheating?

I just pulled the underdrive pulley off my big block stroker. It acted fine on the highway, but would start to overheat as soon as you got into any stop and go traffic. On the weekend I went to the race trck and the car idled for about 15 minutes waiting in line, and it never moved over 1/2 on the factory gauge. My best run was within 1/2 a tenth of the last time I raced with the underdrive pulley.