95 blazer, 2 questions

Took the new blazer for a shake down cruise and had 2 problems, hoping that other owners here might have suggestions or hints:
95 blazer 4 door, v-6, auto, elec. transfer shift, 3.9 gears, 130,000 miles..
It passed smog no problem and seems t run like a top... went on a weekend trip and climbed some mountains for a test and it got hot on any grade that slowed us to under 45 mph... I tried to keep it between 2,500 and 3,000 rpm and that helped, but had to turn off AC and watch the gauge like a hawk..
Seems to run normal between 190 and 200... went as high as 225 or 230 up a 26% grade (didn't really have time to look on the twisty road)
Is this normal? I know that it didn't approach the red area, but I'm concerned about the cooling system.. OTOH, I don't want to put a lot of money into it if this is normal..
3 or 4 times this weekend, over about 700 miles, the "check engine soon" light came on... usually accompanied by a rough, wandering idle... If you're at a stop light in drive, it might run up and down from 500 to over 1,000 rpm and back.. Acceleration and all seem normal and it got over 20 mpg this weekend...
The light it goes off by itself... I checked the gas cap and it was ok.. the manual says that it's usually an emissions issue but it passed smog the week we took the trip...
Any help appreciated... thanks Mac
https://home.comcast.net/~mac.davis https://home.comcast.net/~mac.davis/wood_stuff.htm
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