2001 Toyota Tacoma Problem shifting from 1st to 2nd

I have a 2001 Toyota Tacoma Xtracab with a 2.4 liter 4-cyl. auto trans w/ 4 spd O/D. Just recently, it started having problems shifting from 1st to 2nd gear. I have drained and changed the fluid and changed the filter twice and last weekend took out the valve body to inspect it. Saw no large metal flakes or anything out of the ordinary, was using an alldata subscription to troubleshoot. Put everything back together with new filter and fluid, same problem. If i start off and accelerate very gently, it will shift from 1st to second fine between 1000 and 2000 engine rpm. However, if I accelerate from a stop it won't shift until 3200-3400 rpm. Is this a 1-2 valve problem, governor valve problem, throttle position indicator or what? I'm really frustrated although it drives fine in other gears and overdrive, and will downshift when the accelerator is punched. Thanks for any help.