You can't engage the autopilot below 200 feet AGL!

You must advance the throttle above 1.00 EPR - Actually I advance to almost full and the auto throttle will keep me from exceeding the T/O EPR limit. This is with A/T armed and EPR selected to ON. VNAV, LNAV & CMD - all off.

Make sure you trim your aircraft properly... Most of the time my C of G is around 35 - 40% and that is usually with a 3/4 load out. I have never had a problem when trimmed within the green band...

When you rotate, don't climb more then +20 degrees and trim nose down once you are around 1500' to maintain about +7 to +10 degrees, and that is usually when I engage the VNAV and LNAV. Sometimes I wait a few more thousand feet before using VNAV.

I hope this helps.  ;)