Sounds like you got the pesky torque converter lock up. take off your valve cover and check for hamsters. also flush and replace your headlight fluid...
just joshin ya. but yea sounds like the torque converter is just locking up and putting you in granny mode.
Do the unlock mod and see if that helps at all. it will allow you to pull andpull and pull.
ph33r me im l33t: I'm like ph33r me im l33t: petting the tranny cooler ph33r me im l33t: it looks like a baby intercooler ph33r me im l33t: it's so cute ph33r me im l33t: seriously man ph33r me im l33t: it's like ph33r me im l33t: I flow shit ph33r me im l33t: what now ph33r me im l33t: I cool that bitch down