I have an SS with a 3

I have an SS with a 3.8L. I recently replaced the cam shaft sensor replaced. The car ran great for about 20 miles, then started hesitating when giving it fuel. I think it actually backfires. The tach was reading 4000 and the car rolls about 10 miles an hour. I luckily got it straight to the shop for repairs. I had recently had replaced the catalytic converter, and had a full tune up before this problem. We now have replaced the mass flow sensor, idle sensor, coil packs, put another cam shaft sensor in, ( in case of defect), checked fuel pressure, and now have replaced the computer located under air handler. Car will start and run well when cold, but within 10 miles will start these problems all over every day. There has been no codes registered on the computer since the cam shaft sesor was installed. It has been at the Chevy garage now for over a week still without any solution. Can you help from anyones problems that were similar. Kit