99 s10 tranny slipping

I have a 99 4.3 Zr2. My tranny shifts fine when I drive around in town, but when I have been driving down the highway for awhile and go through a town it slips from 1 to 2. I have changed my tranny fluid and filter and even tried using lucas...no luck. This doesnt happen all the time but when it does I can stop at a gas station, turn it off for a couple minutes and when i start driving it again it is perfectly fine! My parents also suffer from this on their 00 Blazer. Any Advice?

What is the mileage and transmission service history?
Is fluid level full hot in parK?
What kind of fluid was used in it?
Any check engine lights?

You need to get it on a transmission/engine capable scanner and read out all sensor and shifting and slipage data.
Check for codes engine/transmission in scanner and post back code no.
MT

What is the mileage and transmission service history?
Is fluid level full hot in parK?
What kind of fluid was used in it?
Any check engine lights?

You need to get it on a transmission/engine capable scanner and read out all sensor and shifting and slipage data.
Check for codes engine/transmission in scanner and post back code no.
MT

The mileage is 112,000 and i do not know about the service history. The level is full when hot and i used dextron 3 mercon with lucas. No check engines lights have come on either. Im about to start Auto Tech classes at school so maybe i will have it hooked up to the scanner

Do not wait to long.
Slipping can burn up the transmission fast.
On a good transmission do not use additives.
Just good pan drop and new filter.
Was there anything like clutch material or metal or black grit in filter or pan when you changed fluid?
MT

i searched tsb's on motor alldata, and it seems that chevy has had recent problems with their tranny valve bodies, it senses it slipping and cranks up the line pressure. looks like im gonna invest in a new one