Trans is in a 2000 gtp, 181000 miles, refreshed @ 100000 with hardened 4th shaft. The problem is when I manually shifted to 1st while doing 10-15 mph then stepped on the throttle, racing my nephews 05 gto from a roll, it slipped right up to 5000 quick. When in drive it runs fine no slipping at all. Shifts fine and all. Just a problem in manual 1st. Any ideas?