new problems after cat was replaced,PLEASE HELP!!!

here's what happeened in chain of events,maybe someone can be of help?3.4,, 94 grand prix engine, after taking cam covers off and replacing timing belt
and properly timing the cams,along the way replacing back o2 sensor,and dropping new set of plugs in,replacing a melted pcv line,started car up,ran great all cylinders up and power galore,but of course cat converter was making awful noise,all broken up inside,although it was making noise before belt gave out it was somewhat louder now after many starts working on car and several times timing out till put in correctly,at the gas station while fueling up,i had hood up and hand throttled it a little checking up on it and heard a loud hiss for a few seconds(like a air release)then the engine began to idle roughly,it was now missing and the cat converter really got loud and noisy,i took it the next day to have tehe cat replaced and somewhat did a diagnosis on the car,it appears no vaccum leaks (that i can detect anyway)it appears three cylinders are missing(and each cylinder is on a different coil pac,there isn't two on the same pac like a bad pac,)this can be deceiving because the car is running good enough as if only one cylinder is dead,but upon driving the car seems to smooth out and a lot of power driving,just at idle is the miss(s) noticable,you can hear and feel the popping and missing out the tail pipe too,another thing i noticed too,,upon throttling fast and consistant under the hood,the rpm's will want to keep cutting back at a certain point. any ideas what the hiss sound was upon throttling then the rough idle started?????,,thanks
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lol....john

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