sometimes when i get in the car and start it and put it right into reverese/drive with my foot on the brake the engine will almost stall and the quickly rev up and creep. this happened a little when i put gas in it and then started to drive it. and now it happens more often after i reinstalled my catless xpipe. in order for this to not happen i have to start it and let it run for like 10 seconds and then put it in gear. any ideas what this could be or if its hurting anything?