Jared Pool

My 01 Z28 is having difficulty. you put it in gear and take off it takes to 3500 RPMs to shift before kinda jolting into second. After that it is fine. Fluid level is good. i have a 3600 stall and an aftermarket cooler. Any input on this? From time to time you can take your foot off gas pedal and hit it again and it shifts fine. if i have to replace the trans, what brand trans and stall are good. Car puts almost 400 torque and horsepower to the ground.