You could have been running out of fuel. That is a very high load condition and the car may need some fuel help to keep up with your mods up top under that condition.
Green 2000 SVT #1166
mods-K&N open air filter & Custom heat shield, Custom y pipe and 30 in resonator,DMD, arm rest heightened
http://www.geocities.com/savinoss/Cars.html other cars-89 sable 197000 miles and still going-Wifes car 99 Mystique V6 5 spd
