Problems with engine fuel system or are flow management

[Bakerm1]

I own a 1995 XLT with the 4.0L in it and recently I went away to college so now it will sit for a long period of time then I will drive it back home or something that is a few hours away. What my problem is that after driving just at sixty to seventy miles an hour for a while then when I need to go up hills I shift down and the motor will rev at about three thousand for off and on for half an hour. About half way through this I will start to have problems, the throttle response just acts like there is a three to four second delay. It still has about the same amount of power, and when I come to a stop at a light it will idle at only four to five hundred revs instead of its usual seven to eight hundred. I dont know what part, I have recently replaced the IAC valve and was thinking next would be MAF, hoping its not the fuel pump, which isn't making noise, and will also give the filter a quick change to come spring break. Any thoughts would help thanks