2002 Frontier auto trans problem

My 02 frontier is having problem shifting in gear when you push down on the gas. You can tell it want to go into hi gear but i don't. I can't even pass anyone. I has 95000 miles on it. The last time i had this problem was at 90000 miles. The tech told me nothing was wrong then. I need to know what will cause it to do this. Some days it do it and some days it don't.

check out

http://www.clubfrontier.org/forums/i...eferrerid=1451 (http://www.clubfrontier.org/forums/index.php?referrerid=1451)