The thread where guy was having similar problem is closed now: https://blazerforum.com/forum/2nd-ge...on-surge-2832/

His problem was surging when going up hill I think..

I have 1997 Blazer 4x4 4.3 vortec. I usually can solve most problems; I'm fairly mechanically inclined but this one has eluded me for some time. Blazer runs fine at all times up to 3rd gear. When using OD (4th gear) at 50+ mph it likes to downshift for no apparent reason. There is no load, I'm usually going flat or downhill. The tach will also jump up and down and I can feel a surge being transferred to the drivetrain, on and off (like its going in and out of OD). It downshifts when going uphill as it should and stays in the gear it should be in. I've just been driving with the crappy gas mileage and compensating: when rpm surges on the highway I'll just let off the gas, coast and it'll stop, if the hill is big enough I'll put it in neutral and it'll sit at 1k or less going down. This saves me a lot of gas. I can give it gas and speed up quickly and will also stop the fluctuating, most likely because I'm in 3rd gear (again judging by RPMs sitting at 3-4k. I can work around the problem this way but it's super annoying. I just want to drive like normal. I get the feeling I'm not the only one who has experienced this, any ideas would be appreciated. Thank you.

I'm going to do this MAP sensor test tomorrow:

And this TPS test:

Useful videos btw if you wanna share them if hasn't been done already.

 

This sounds like a vacuum or sensor problem. The vehicle is able to sense when you're giving it more gas and needs to downshift for more power. I think whatever this sensor is, is where your problem is. I wish I had more know how on the systems in our 4.3s, but I don't know any specifics on this one.

 

I've been having the exact same problem. Did you ever figure it out?

 

MY 1997 4 wheel drive has the same problem. Any suggestions?