[BR3W CITY]

Trans level cold is useless, needs to be warm and have been in P,R,and D to make sure every fluid path has taken fluid. If the level is good then, and the fluid looks good and doesn't smell burned or like hot sh!t, move on.

Do you know what size tire the ECM was told you were running? A significant differential between the input size and the actual size will not only throw off the speedo, but more importantly, throw off the shift points and the lockup draaaastically.

As an example; my '66 runs a 325/50/15 drag radial. Thats around a 28" tire. I had to toss on my old front rollers, a tire thats only 225/60/15 or a 25.6" tire. That difference caused hugely annoying issues with the VSS/ISS reporting speeds @ rpm that didn't fit the computers logic. This usually means things like ending up locked up in 4th only going 40 and lugging like all heck....or the opposite, randomly unlocking and re locking, almost feeling like your going up and down a gear constantly.

If the tire size in the tune is correct, then (assuming we've made sure no other mechanical problems are affecting it) we would start to look at the trans tune setup. Most tuners don't really touch trans settings when they should, and 75% of the ones that DO mess with them do it wrong. If it does come down to troubleshooting the tune, then simply reflashing/resetting a base GM trans cal should sort things out quickly.