99 Blazer 4x4 - Intermittent hard shifting AT w/ code

[Tangent]

Every once in a while my wife's 99 Blazer (4.3, 4 speed auto, 4x4, ~100,000 miles) will start shifting very hard as if it had the shift kit out of a drag car in it. Shutting the engine off for ~1 minute seems to reset this condition but it's getting more and more frequent and today my wife had to reset it 3 times on a 25 mile drive home. It will sometimes set a code 1870 (generic transmission code I believe), but not always. I've tried desperately to find a pattern to when it happens but just when I thought I had it narrowed down to being temperature related it went for months without doing it including a stint in stop & go traffic in 115* temperatures on the Vegas Strip...

A search found a few similar problems but not quite the same except for one from 2003 that sounded identical but had no posted solution.

I've contacted a few local transmission shops and have gotten over-the-phone diagnoses ranging from needing a complete rebuild to just needing a shift kit that contains a redesigned part to replace the one that caused this (according to him) fairly common problem. Has anybody seen this problem and its solution yet?

**EDIT** - This happens on all shifts up or down but it's of course more noticable in lower gears and when accelerating.

[BlazerLT]

Have you ever replaced the fluid and filter?

[Tangent]

I was actually at a transmission shop with it last Friday after I discovered to my horror that my wife had never done that before I met her. The shop recommended that I wait until their slushbox guru was in and have him drive it before changing the fluid on the premise that at this mileage the old contaminated fluid might be all that's holding it together. On the plus side though the fluid does not smell burned and it's still a nice clean shade of red. At least what I pull up on the dipstick is clean. It's never even needed a top off of fluid...

I'll be taking it back next Friday for the "guru" to evaluate.

[BlazerLT]

Originally Posted by Tangent

I was actually at a transmission shop with it last Friday after I discovered to my horror that my wife had never done that before I met her. The shop recommended that I wait until their slushbox guru was in and have him drive it before changing the fluid on the premise that at this mileage the old contaminated fluid might be all that's holding it together. On the plus side though the fluid does not smell burned and it's still a nice clean shade of red. At least what I pull up on the dipstick is clean. It's never even needed a top off of fluid...

I'll be taking it back next Friday for the "guru" to evaluate.

Go and get the fluid and the filter changed. Transmission shops just love to have you replace expensive components when it could be just the usual maintanence needed.

By on guard, tranny shops are known to be crooks.

[Tangent]

I suppose I have nothing to lose beyond the few bucks that the fluid change will cost me at this point... If I leave it alone it's getting undriveable very quickly. If I do the fluid / filter I at least have a chance of solving it.