Getting this info second hand. Here Goes.
Blazer 4 door Automatic W/144K miles on it. Shifts fine, does not show any sign of a Miss while under load. But drive at a constant 40 miles and hour and you get the feeling of a gust of wind hit the side of the car. the Tach does Drop in RPM. You can accelerate out of this and it will drive Happy at 55 and up. At 35 and under it is also happy. No Codes. I think I am ready to say that it's not an Ignition issue. Maybe the Lock-up Torque converter is bouncing in and out? Trani was never flushed nor has the filter been changed. IF it's the converter, what are the chances an oil change would help?

It almost sounds like this issue BUT this post was never really resolved.
https://blazerforum.com/forum/engine...erdrive-90819/
Any thoughts?

 

Sounds like normal operation of the TCC, just like I answered in the post that you linked. You simply can't resolve a non-problem. Solution is if you don't like the TCC going in/out in the 40-50mph range, then drive it in third gear. TCC locks up at 35mph in third and is not so likely to cycle in/out. In OD it should be waiting to lock up at 47mph as you increase speed. Will stay locked in until maybe 42mph or so on a coast. But in third, it will stay locked up much better. What's the difference. 2000 RPM in third with the TCC locked or 2000 RPM in OD with the TCC unlocked. OD is for highway speeds.

 

She's been driving the same road where this occurs for years. this is new. I was hoping it wasn't a bad Locking Convertor. She called today. She got a Code 1870. apressure issue causing the Slippage, so that's good. The trani doesn't have to be dropped. A new Valve body will most likely fix it.

 

Read this carefully about P1870 and make sure repairs are not the patch jobs that cause other problems. Also know that if PWM/ECC operation is modified, you WILL notice differences in how the TCC applies. I have been bugged by this after Precision Transmission "rebuilt" my 76K low mile transmission for an unrelated issue and I paid $2500 for the privelege. Honestly, I think they just replaced it with a rebuilt hi-mile POS that had a TCC modification done. Now I consider the TCC apply to feel severe and the ECC function apparently does not work at all.


20 Years Later: What?s New with 4L60-E PWM Modifications - Sonnax

 

I had the same issue in my 98 two door. Turned out that the trans fluid was just a hair low and adding some fluid fixed it!