What would a 'slip' feel like while driving?

[Shrewz92]

1995 Century, 3.1L, 4T60E.

Service Engine Soon light is on, and will click off after couple of days. Scan @ Advance Auto Parts generates a P1870 code. I've read this could be slippage of torque Converter, or a valve blockage. I'll honestly say I've NEVER felt an auto transmission 'slip.' What would that feel like when cruising at highway speed (55-65 mph)? Would that feel like a 'stumble?' Would that feel like a 'jerking' motion?

Additonal Notes that may help:
I know that I have trans fluid in, actually to above top of cross-hatch on dip-stick.
Doesn't look like it's frothing while idling and it being checked.
Fluid is healthy reddish pink.
When purchased, I added about 1.5 pints of high-mileage trans fluid to what was in existence in the system, as it was reading near bottom of cross-hatch.
Occasionally, When putting shift-lever in Drive, there is no engaging of gears, until approx 15 - 25 seconds, then it smoothly enters 1st, and shifts nice and smoothly through remaining gears. Would too much fluid situation cause this 'non-engagement?'
If it's a valve-body needing replaced, is this something that one can replace from outside, or from under trans with pan off?

[Shrewz92]

Update:
Weekend of 8/1: Changed Transmission Modulator and found crack in vaccuum hose entering modulator. Replaced and no hesitation, now, before engaging 1st gear from standing stop. After change of fluid & filter & Modulator, no check engine light until approx 108 miles after. Still P1870. Is there a deeper diagnostic scan that will show what that code is caused by?