4L60E problem?

[5150Watts]

97 K1500, 5.0L 155,000miles. 33x12.5 tires if that matters for this

The truck shifts harsh and jerkey some times, but some times it is smooth. Notice it gets better as truck has warmed up. It also sometimes over downshifts when I need a little extra power by dropping down 2 gears instead of the normal 1. Taking it too a few tranny shops and they found really no problem. One said that the engine is lacking some power causing it to do this. I don't know how they came to that conclusing. It is a bit slower then it use too be but, I think that its the age and mileage. Pulled the pan and replaced filter and fluid. No sign of any damage at all plus very little shavings on the magnet. Fluid was in pretty good shape too. Also, no trouble codes currently or stored in memory. So, what do you think it could be? Valve body? TCM? Any help would be greatly appreciated.