CEL on or any codes stored? a code can be stored without a CEL, but will show up when scanned.
Any particular variable that may be present when the trans shifts?
Hot, cold, wet or dry weather is usually a factor in intermittent problems.
Does it do it while accelerating hard or casually?
If the trans is failing most of the time the shifts are smooth or lazy, so a hard shift is usually not a problem for the clutch packs (better and quicker contact means less wear) but it could be a problem for other parts if it's too harsh.
I'm not too well versed on common 4L60E failure areas and diagnosing individual tranny parts is hard too (that sounds very wrong :lol:) It could be your servos or accumulator pistons causing the hard shifts. Might also be the valve body or a worn out seperator plate.
It could also be worn out mounts (too much movement during 1-2 shifts could cause crazy results) or it could be u-joints as well.
Best thing to do is get the ECU scanned or hooked up to a proper scan tool and have it taken on a test drive. The tech can watch a variety of variables to narrow down what could be causing the problem such as incorrect line pressure, TC lockup issues, etc.
Then i'd crawl underneath and start looking for worn out mounts and u-joints.