1995 Chevy Astro Oxygen sensor problem code 44

Hi, I have a 1995 Chevy astro that lights up the service engine soon light. I have maintained this van like new, it has new shocks, spark plugs, spark plug wires, distributor cap, new radiator, pcv valve, kandn air filter, I do my regular oil changes and take care of differentials and transmission. A few months ago I installed a brand new emissiions system inlcuding a brand new catalyc converter and all new pipes and muffler including a brand new oxygen sensor. About 20,000 miles ago my light never lit up and now it started to light up whenever it feels like it, It comes on one day and then waits a week or then lights up next month or then lights up today. It turns on whenever it feels like it. I know it is a lean exhaust but I found no vacuum leakage and fuel pressure seems fine. In addition, map sensoer, fuel pump and other sensors have their own trouble code, so it could not be any of them otherwise I would get a trouble code for those sensors, and the only trouble code im getting is the 44. Engine runs beautiful and silent like new. My local mechanic suggested that maybe it was the gasoline that I was using and it could be cause when I used shell gas it would not turn on as much and today I poured in US petroleum and the light was on for quite some time. He also told me to add fuel injector cleaner which I did recently.

PS: Sometimes it turns on and when I accelerate the light goes off. I recently moved the wiring of the oxygen sensor around because some have suggested that maybe there was a bad connection or the wiring was to close to heat or ground of the exhaust manifold! So please help! Thanks!