Stumble/backfire?

[Nixon1]

Ok...the past few days, when I'm cruising in 3rd gear (automatic tranny) at part throttle, around 2200 rpms, the car feels like it's giving me small little backfires here and there....I'm thinking that could be an exhaust leak maybe. When I let off the gas sometimes after being hard on the throttle, it backfires loudly. And also, at cruise at the same rpms mentioned before...every now and then the car will misfire and stumble a few times, or all of a sudden feel like it lags back...like when you engage the A/C compressor and the car feels slower and it takes slightly more gas to keep it at the same speed. What could be causing that? My old EGR valve was frozen up completely, so I have a brand new one now. Also, the TPS, IAC, and CTS are brand new, as well as the plugwires, cap and rotor, and ignition coil. Spark plugs have only been in for about a month but I havent had the chance to yank them and check color. Thanks all.