Lugging / Bucking 5.0 low rpms

[Tom753]

I have an 89 LX 5.0 that has developed an issue that I can't figure out. The car runs great when you start it, and runs good for 20 minutes or longer (longer on cooler days, ie not hot outside), then it will begin to lug or buck pretty violently mostly at lower rpms, so on the highway I get stuck driving in 4th gear a lot (manual trans). The car also is running warmer than it used to, not overheating but running warmer. To date I've replaced the distributor cap, rotor button, plugs, plug wires, and fuel injectors. I've also disconnected the MAF to see if it made a difference it didn't, I also cleaned the MAF with electrical part cleaner, no difference. Any ideas? I'm thinking maybe running lean. The car is mostly stock, except exhaust (no cats, after market H pipe, and MagnaFlow mufflers), and K&N filter. 340,000 on car (all by me), only 60,000 on new motor, TPS may be orginal. Any help would be greatly appreciated.