fc3sna

ok, i have an 88 vert that just had a header installed, but while i was installing it i removed the 5th and 6th acuators and the anti after burn vavle and installed the block off plate with no gasket sealer,..... well the problem then was similar to now but i pulled off the plate and put rtv black on it and put it back on and then about 2 hours later i snuged it down. it ran fine. today i took the car to the exhaust shop today and they straight piped it duel from the header back to two mufflers and when i start it it acts funny it revs in between 1500 and 2000 and when i give it gas it wont go over 2000 then if i put it in gear and start rolling it eventually go's but then cuts out at random and wont go past 5500. it runs fine every now and then and goes to redline fine, when it runs fine it dosent back fire at all but when it acts up it back fires like no tomorrow. any one know what it is, ohh also i have no o2 sensor. i have to go back because they forgot to put it in.
thanks

fc3sna

my cars weird as ****, today it had no problems at all. also i hooked up my o2 sensor so hopefully i get better gas mileage on the longer drives and the fuel filter hopefully will help also. now i have to put in my stero and get a new top......