1996 Mustang GT convertible. 76,000 miles on the clock. Rarely driven. Still has poly intake and will update that when I visit granny next as there is a slight drizzle by the temp sensor, but don't think it's related. I show up to help with things around the house and started driving her Mustang to give it some exercise. Runs great. She said she had to get towed home a year ago because it didn't start, but started later on. It's been sitting, but she starts it up everyweek. She even puts in Lucas fuel injector cleaner at last fill up. So I give it exercise, go through the gears, it squaks em hitting second. Fast, runs great. She drove it to her work and had to get towed home as it wouldn't start, or would barely start then stumble, then go 20 ft then stop. It's now at half tank. Get's towed home, I get fuel pressure guage kit from autozone. Look for 30 Psi and 38 with regulator vacuum hose disconnected and plugged. Boom get 38-40 psi. Car runs perfect. I let it idle, get hot for hour. Drove it let it idle, everything was fine. I take it to walmart that night, totally fine. Next day I run to home depot and it sputters a little. Check engine light comes on. I get the code P0175. Lean or rich mixture detected 02 sensor right bank switch off? I think. I clear the code. Now after 180 miles various driving, no codes. Now here is the problem. At full throttle right at 4,000 RPMs there is a loud popping by the engine compartment. Maybe backfiring into the intake or afterfiring in the exhaust. It is only at 3/4 to full throttle at 4,000 RPM and higher. Pulls hard up till then. I drained tank, clear and slightly yellow fuel. No sediment. Replaced fuel filter. Same thing. Still pops. So after searching I read a guy filled his tank and problem went away indicating fuel pump. He replaced it and took care of problem. I filled it up. Still popcorn pops above 4,000 RPM. Its like clockwork. Sounds like someone popping off a 22LR randomly. I checked injectors 15.6 ohms. I drove it with fuel pressure regulator vacuum off, Mass Airflow Sensor unplugged, everything. It acts the exact same. Light turns green, I punch it, it lays rubber pulls hard, and whamm starts popping at 4grand. I even hooked up the fuel pressure guage under the hood to the wiper, I drove at regular speed getting 30 PSI full throttle it goes up to 38psi and is at 38psi while sputtering. I do not thing its a plug or wire issue. I think the fuel pressure regulator is doing fine, The fuel pump maintains pressure even at full throttle for a while. So I don't think it's that. Last thing, if I drive in first gear and ease slowly on up to 5300 RPM or so no pops, but if I give it full throttle it pops right away. I was thinking catylatic converter, but that usually prevents RPM like a brick wall right? I'm at a loss and I don't want my widowed granny to get took by a mechanic while I'm gone. Planning to try cam position sensor and o2 sensor and maybe hit cats with mallet to see if screen is loose. Please help (sorry for long post, but wanted to give all info).