I'm no expert but.....my 02 cents worth....I had a similar symptom recently, so if this helps, great.
My trans was flairing in the mornings for a minute until it warmed and then ran fine for the day. I think the shift problem you're experiencing can be described as 'tranny shift flair' (do a search here on flare or flair...can't remember the correct spelling).
In my case the ATF was low due to a leak out of the cooling hose from the radiator, and was a simple enough fix. As the ATF was down so low I followed a suggestion to replace the filter and re-fill with Mobil1 synth ATF. What I learned from the experience that could be useful for you:
1) the trans dipstick is NOT easy to read, as say for the engine oil. You might just find you're low on fluid, even if the stick says you're fine. (mine read fine and I actually had ONLY 25% of the required amt!!!!)
2) synthetic ATFluid is AMAZING on an older transmission. My shifts are smooth as silk since the change. I heard that the formulation is enough to fix your problem.
3) Try yourself or have someone increase the fluid pressure by adjusting your transmission modulator....that's what it's for. (again, do a search here, it's quite an easy task). The modulator has an adjustment screw that sets the internal fluid pressure of the tranny, which occasionally needs gradual increasing as the the miles pile up. Shift flair is often eliminated by this adjustment.