Check Engine Light and reduced power

I have a 2008 Escalade ESV and the Engine Light is one and there seems to be a loss in power. I had diagnostics run and it pointed toward the fuel pressure which I replaced. That didn't help. One of the symptoms is when I fill the truck with gas and try to start it, it turns over but doesn't start. The next time I try to start it, it starts just fine over and over till the gas cap is opened and filled up. I have filled it with the engine running, and when the gas cap is taken off, the rpm's slightly decrease but recover while the gas cap is off. If I go that route, the engine will start just fine over and over. It seems that pressure is relieved but not recovered until the fuel pump creates the pressure. Please let me know what suggestions there might be on what to replace or try next. Thank you....