I have a 2002 trailblazer LS 4X4. Cat back exhaust system, Throttle body spacer and K&N air filter in stock air filter housing. I bought this new in 2001 and these items have been on it from almost new. It has 136000 miles and does not get driven a lot. About 3000 miles per year but gets driven some every month. From new it has had a slight hesitation at off idle and still has it; this is even before the spacer was installed. Dealer said that was normal B/S. The problem I need help with is that it started running rough at idle and at speed a few weeks ago, but does not do it all the time, seemed like a fuel issue but no check engine lights, no service engine soon lights. The truck has had all maintenance done, oil changes at 3000 miles using a Valvoline max life 10/30 I live in Tucson AZ so it’s hot even in the winter. I cleaned the throttle body; it was not gummed up that much. Checked for vacuum leaks using carb cleaner, did not find any. Changed the fuel filter even though it did not have that many miles, it was about 5 years old, it had clean fuel, smelled fine and the filter was not plugged. It has original fuel pump and fuel pressure regulator, I have always filled up before it got to ¼ tank of fuel. I checked the Fuel pump pressure at the fuel filter and it read 52 PSI even when truck was missing. Specifications are 48 to 54 PSI. I get a slight burnt gunpowder smell sometimes. I have a basic code reader and was not able to pull any codes. it was still running rough so I decided to take it to a shop that I have used in the past but it was a few days before I could take it. Of course it ran fine all the way there but I had them use their diagnostic equipment. This is from their notes. (found a P0300 code on cyl 3 and 4 at startup will have 2 to 4 misfires on Cyl 3 and 4 between idle and 1500 RPM but misfires are at PCM only and cannot feel at idle or driving) they pulled the 3 and 4 plugs and were fine some rust in number 4 tube probably from the rain at the hood cowl. I had them go ahead and do a fuel injector cleaning while it was there. The plugs were installed at 114000 miles at the dealer after trying the E3 plugs and it running bad, I am assuming that they are AC Delco since it was done at the Chevrolet dealer. Also had the water temp sensor and the thermostat changed at the same time. I had my wife drive it every day for a couple of weeks and a couple tanks of gas with no issues except that it seems and sounds like it is pinging on medium to hard acceleration, it still has good acceleration. Then one night it started running rough but not all the time. I have stopped driving it every day in case it is something that may harm the catalytic converter if it hasn’t happened already. Sorry for being long winded but I wanted to give as much information up front. Please Help