4T60 Transmission 'bump' when going 3 -> 4

Wow - I'm amazed at the wealth of information here in these forums...

I have a '91 LeSabre with 211K miles on it; engine runs well. Just noticed some disconcerting behavior from my 440-T4 / 4T60 transmission this morning - with the engine under some load it tends to 'bump' a time or two after entering 4th gear. I have been able to reproduce the problem. For example, I would be driving along, accelerate to just over 45MPH, tranny goes 3 -> 4, then start climbing a hill and apply a bit more gas, and transmission gives a harsh bump or two but stays in 4th, like it is perhaps trying to shift 4 -> 3 and then changing its mind? I have not noticed this behavior at all before... and am a bit concerned about the tranny....

The tranny otherwise seems to work great - fluid looks/smells OK and tranny shifts fine including downshifts to 3 and 2 at interstate speed. I am curious about the 'prognosis' for this kind of symptom - is it just a misbehaving valve body?

I suppose this could be non-transmission-related, but it definitely feels like it is coming from the tranny.

Anyhow, I will probably backprobe the TCC apply circuit and the 2/3/4 gear indicator circuits just to get a better picture of what might be occurring, but any thoughts would be welcome...

Quick additional info here - have tapped into the TCC apply circuit and 2/3/4 switches, and have LED's to track what is going on. BUT so far have not been able to get this machine to malfunction again (it was repeatable before). Will try again tomorrow. I also found that at 45 in 4th w/ TCC, when going up a hill the first thing to happen when I step on it a little seems to be the TCC releasing, not 4 -> 3 as I had surmised.

Also found this thread over in the Olds section:

http://www.automotiveforums.com/vbulletin/showthread.php?t=174521

Very similar to what I experienced but my alternator seems to be OK.