My 1991 yj has been a good jeep for me but is starting to give me a problem. When cruising in 4th or 5th gear at about 1500-2000 rpms I can hear a whistling noise coming from the engine area. I have recently put on a cold air intake and thought maybe this whistle was from it drawing air but now I'm not so sure. The jeep will every once in a while feel like it totally loses power when cruising at 1500-2000 rpms. If I mash on the gas pedal it will regain power but then when I let off the gas again it will lose power. It doesn't always have this problem just maybe once or twice every 15 min trip. What could this be? I have never dealt with this before. Any help would be greatly appreciated.