Also finding a few time I open it up, shifting up either manually or leaving it in D...and sometimes at WOT it does not shift. Rev limiter of course helps, any ideas why it does not shift??

 

Usually it is not enough oil going to the 3-4 clutch pack, or the clutches are starting to "go away". This is very common on 4L60E's. How many miles on the unit? What is the condition of the fluid? Any recent modifications that coincide with this happening?

 

Hey Dana, trans is an GM rebuild, has about 60K miles on it, only mod is the HPP shift firmness option. I'll re-chcek the fluid, service was done less than a year ago