2000 LS1 - Stock (with Predator program) - 174k miles

While upshifting during slight acceleration, my RPMs will go up and the car gradually goes into the next gear. (no shudder or jerking) While rolling along at highway speeds, if I "floor" the accelerator the RPMs will shoot up and the car slowly, but smoothly, shifts into passing gear.
However, if I WOT from a standstill, the shifting seems fine.
I only suggest the torque converter because after a new tranny install on my friends car we experienced a similar problem. He had not put in enough fluid to fill the converter and it took high RPMs to get the converter to work until fluid was added.

Thanks in advance,
Jason

 

try a stall test, check your fluid level, you also might have a gummed up VB