91 cav Huge prob need help PLEASE!!!any help would be amazing

Alright after sitting hear reading most of the threads i noticed alot of people had the same problem soo im gonna try and explain mine

alright i have a 91 cav z24 standard, now i had a problem about a year ago with my car stalling out at stops of any sort even backing out and going into drive stalls.. so this problem just started again today the thing is its only doing it now that i put my new muffler on i had a "perforance" on on last night and it worked fine and same with my old air filter got changed last night.. so i figured it was on one of those problems so i just simply put all my old air filter stuff back on still ran like crap and going throguh the work of changign the muffler back sounds kinda dumb. when i started my car today (when the problem began) it started rough then mabey 5 seconds later the idle rpm goes way up then back down starts to seem lik its idleing good and running smoothly then i get into drive drive down the streeet and go to stop car goes off and after im stopped it will start right back up and the problem starts alll over again. So it took me forever to get home and well driving i just downshifted everywhere to keep my rpm going and so i didnt have to take it out of drive and i made it hope but its retarded also im noticeing since this problem started again my cars been running kinda hot like reallly noticeable hot and in general this problem sucks i just dont have the money to get a garage to help me out. That is one of my worries the seocond is my check engine light is on has been on for a long time but it seems to be weird when my check engine light is on my car runs like crap crappy exceleration and really bad fuel economy runs alright at high rpms but anyway i got the code read at school and my one cylinder isnt firing i have new plugs and wires and we tested for compression and sudd and i still dont know whats going on i find it weird how it kinda temporalily fixes itself then goes back to running like garbage anyway PLEASE any help would be wonderful i cant take it any longer i dont have the money for a new car so im pretty much screwed
THANK YOU who ever can shed some light on this problem/problems

Could be a bad Coil-Pac. Sometimes when cold they can "fix themselves". OR You may have a sticky Iac valve or fuel injector messing up your fuel/air ratio randomly. Does this car ever back fire? If so look at the coils and wires. Good Luck.

Alright yes it does back firewhen i downshift or even sometimes when i just rev the engine i was told it might be a fuel injector problem but im not sure and as far as the IAC valve ill try replaceing it or whatnot not to sure where its located but ill try and find that out
thankssss