The cause is either valve body or TEHCM (or both). Broken down, they are as follows:
- SS1 failure (replace tehcm)
- Clutch select valve and/or bore worn (install Sonnax zip kit then run fast adapt relearn)
- cbr1/4-5-6 clutch regulator valve (install sonnax zip kit, run fast adapt relearn)
- check ball #1 worn, sticking in plate
- cracked filter (not common)
Take for a test drive with a bi-directional scan tool and monitor the live transmission data feeds for anomalies.
Then drop the pan and inspect the bottom, look for debris. Inspect the filter for cracks (replace if so). Test SS1 for continuity and note resistance - if it fails, replace the TEHCM.
Remove, disassemble the valve body-TEHCM complex. If you have the tooling, perform a vacuum test on both halves to locate any under-performing valve trains or locations where there's insufficient vacuum (my standard is 15" for all valve trains). Disassemble and clean the valve body castings in the parts washer/jet wash...clean all the valves, springs, etc and lay them out in the proper order for reinstallation...Inspect the spacer plate for wear at the check ball locations - pay close attention to CB1 and CB5. If CB1 gets stuck, you lose all forward movement whereas if CB5 gets stuck you lose reverse. Replace the spacer plate (you can grab one from the Parts dept in any GM dealer). Also, purchase valve body-case seals for the pump and center support since you can replace them while the valve body is out. You'll need a Snap On 10EPL socket to remove the six (6) valve body assembly to case bolts. Torque spec is 71 inch lbs; same for the valve body halves and IMS bolt. The two small 10mm bolts for the speed sensors take 90 inch lbs.
Install either the Sonnax Zip kit or Transgo Reprogramming kit...Both will come with drop-in replacement valves, springs and other parts to rehabilitate the valve body. Reassemble the valve body complex.
If the TEHCM is determined to be good (i.e. no faults) then install Transgo's TEHCM service kit (has diaphragms and seals for the TEHCM pressure switches).
If you don't have any of the above or aren't willing / able to do the above, take to a transmission shop or a GM Dealership service center...They will do all of the above and let you know what's up.