start van runs ok. take off up to 35 mph ok. give gas at pedal to go faster and ping and or knock is noted and lost of power. slow down below 35 and runs good. no check engine light is on and light is ok. pulled intake and put vacuum on egr valve valve holds good vacuum. already replaced timing chain and gear before so this was not the problem. did a tune up when timing chain was replaced. I am thinking the cat is almost stopped up and to much back pressure on accl. is causing the problem. but I don't know. What do you think.