1998 s10 blazer misfiring/stalling problem?

My blazer has been stalling bad when I drive normal, but when put the peddle down to the floor it accelerates perfectly so I kno it's getting fuel. I just changed the spark plugs, distributercap and rotor. The check engine light is on so I hooked it up to the scanner and it said random cylinder misfire, and anthoer code saying activity bank. I noticed a vacum line Goin to the throttle body was rotten so my teacher(which is ase certified) took the rotten line off and stuck a skrew in the line to block it. When he did that it ran perfect, but the next day it was back to the same ol' stalling problem. By the way this all started when i got a new exhaust system put on. I look and the oxygen sensor is on so that not the problem I guess. Another thing is when my engine gets a lil over temp 210 it runs good?. Can someone please help me out asap!?!?