My transmission is jolting into second gear.

Hey guys I have a 98 silverado 5.7 litre 1/2
ton 4x4 that I bought it off a guy back in
February for a pretty fair price. He said he
dropped a new tranny and engine in it
10,000 miles ago and topped off all the
fluids to the right levels (which i checked,
he did). Anyway, when I test drove it, it had
no problems at all, shifted smoothly. But a
few days later it started jolting into second
gear pretty hard every time i accelerated
after coming to a stop and the check engine
light came on after it started doing it. I've
seen posts from people having the
somewhat similar problem, but here's
where my situation is different. It starts
doing this ONLY when I have been driving it
around for about a half hour when its
warmed up. But if i turn it off and she sits
for even just a half hour and start driving
again she'll shift smooth and the check
engine light turns off (but she'll start doing
it again after a few minutes) of driving. The
engine temp is fine, I checked the fluid for
any signs of glittering, difference in color
and smell but it was fine. I took it to
autozone to see if it threw any codes, sure
enough it threw a tranny code. So just to be
safe I went ahead and changed the tranny
fluid and topped it off to the correct level
and fluid, buuuuut she keeps doing it. My
buddy who's been helping me is a certified
mechanic and said he doesn't know
because hes not too familiar with tranny
problems. I cant afford to get her
diagnosed right now, so I just wanted to see
if anyone had any ideas? Thanks