87 s10 blazer no power

Good Afternoon!
I have a 1987 S10 blazer 4x4 with the 2.8L v6 what has been happening is that she shifts fine from 1st to 2nd but when she hits 3rd she looses power especially going up hills she won't go above 20mph. this is with my foot pressed right to the floor on straight aways when she goes into 4th she picks up speed again. When I bought her the guy had headers & a Holley carb on her which she still has. I have adjusted the tv cable to the tranny. I also find that she does this more noticeably when my temp. gets up to 210. She also stumbles at that temp. I have changed the radiator put a 180deg thermostat in her & coolant temp. sensor. I still think it's the tv cable but could be barking up the wrong tree!. Any ideas would be greatly appreciated!. Thank you for your time.

it could be several things, but it sounds like a cat,,converter.

take it to an exhaust shop to have it checked for back pressure.

or if your a mechanic put a vacuum gauge on it and rev it up..the needle should come down fast to about 20in. if slow comming down that means bacl pressure...re. cat. or bent pipe.