2002 Cadillac Escalade EXT acceleration and decrease of rev

Ok I bought the truck and drove it for about 150 miles that was like in two separate trips and then it turned on the service engine light, it gives me the code p0420 two times the truck has turn downs and it runs good and accelerates good too, but the problem is that when I accelerate a little bit (park or running) the revs go down to like 250 thats like below the first line up to the middle and it feels like if it wold turn off but it doesn't and the truck doesn't vibrate or shake at all when im running or when im park


(in addition when I plugged it to the computer to get the codes after the p0420 it also said misfire, fuel comp, catlyst, evap, egr, monitrs, 0 inc, 8 ready, o2 sensors, o2 htr) was this cause by the turn dows or how can I solve this and what migth have cause it?