2000 ZR2 Trans problem

I have about 150k on my zr2, and the last coulple of days it has not been shifting into overdrive this problem is intermitent, and happens mostly when cold, is this the lock-up? Any help?

Welcome to AF.
Check for transmission codes and shift data with a transmission/engine capable scanner.
Post back any codes and shift adptive data.
Engine not running right will cause transmission not to shift right.
How is the fluid level and condition?
Has the transmission ever been serviced?

Some have a delay on OD when engine is cold.
Does it take low and reverse good when cold?

I flushed it once after i got it, like 25k ago...gonna do another flush, and change filter tomorrow.
I will check for codes and let you know.
Fluid level is good,
Noticed the truck is bucking when cold when shifting from 1st to 2nd....
Has been setting O2 coded intermittently...
truck has 150k on it...i do not abuse this vehicle, all highway...

I flushed it once after i got it, like 25k ago...gonna do another flush, and change filter tomorrow.
I will check for codes and let you know.
Fluid level is good,
Noticed the truck is bucking when cold when shifting from 1st to 2nd....
Has been setting O2 coded intermittently...
truck has 150k on it...i do not abuse this vehicle, all highway...

Fix the engine running problem for sure.
The engine has to be running good for transmission to shift good.
If that does not take car of it get it on a good transmission scanner and check it out.
Good Luck