Low end power loss

[Jason Thatcher]

I have an 02 Tahoe I bought from a buddy about year ago while I'm building my lmm 3500, right off it started dieing of after 30 mins the less and less til would barely run 5 mins, I did all the usual run codes even though no check engine, the bottom end was freshened up and complete new too end about 40k before I bought so first thing u did was upgraded clogged fuel filter and switch out plugs, next upgraded plug wires then cleaned MAF, and cold air, astly I took out cats and added a 2.5" in and out fkowmaster with turn  down just past axle, still no fix come find out weak fuel pump, after that fix this looks bad but I thought exhaust was 2.5" but it was 2.25 so exhaust came off 1  on both sides before I got it fixed for most part, clamps still leak a little just now got my welder going to weld, and also the cold air slip off twice before I put box back in. I had check engine light and exhaust leaks but no communication with scanner. Gas mileage had gotten down to 12 hwy, and I would literally have to floor it to get rpns up to get it going, like it was abouy to die, but what I don't understand is no matter how many times I took battery out  for day at a time computer would never recet but as soon as I got the scanner to work earlier and cleared all five codes, 1 for MAF and 1 for each 02 sensor and test drive it, it's running better than it ever has, are the computers that finicky on these motors?