Y2KSLPSS

I had a p0440 code a couple months ago, I changed the gas cap and it never came back. But, now after the car gets warm it will start missing & sputtering and running like crap. The problem is it is randomly happening and never throws a code. I know the computer runs the test only on "cold" start-ups(per the gm service manual) and from what I've read on searching this topic, this is a B@#*! to troubleshoot. So, what could be causing this and what might be a good scanning tool to test the sensors/solenoid's? Thanks for any help & experiences in advance!!