I have a 99 cherokee sport straigh 6 4.0L 4X4 standard gear, and i was experiencing similar problems as you. During a year, I took off the fuel tank, cleaned the micronic filter inside of it, cleaned the injectors, changed spark plugs and cables, and everytime i changed or cleaned something, started to work better for a while, and then again, the problem. a Few months ago, a mechanical told me it was the TPS, so he replaced it and reconfigure the computer, so the engine felt very good (stronger), but a few weeks later, it began to give me trouble starting, i had to push all the gas pedal and give a lot of start. So, i had to reset the ECU and the engine began to start right, but the iddle and acceleration problem came to again. The last thing i did was to clean the IAC valve and amazingly the engine began to work fine, i thought all my problems were gone, but a month later the ptoblem started again. i cleaned the IAC valve again, but the problem just got worse, after a few minutes of not running good the engine would not responde to the accelerator pedal. i had to stop the engine, wait for about 30 seconds and then start it again and that would give me another few minutes. I thought i messed it up in the last cleaning to the IAC valve and wanted to replace it, but i read your article and a person was suggesting you to check your vacuum lines. so i checked mine, cleaned one by one, and in the vacuum hose that goes on the rear (next to the cabin) the hose has a plastic device that is supposed to allow air to go just on one side, but mine was dirty and it was allowing the air to go on both sides. Some carb cleanner, and a Little shake were enough to make it work properly. i reconnected everything, started the engine, and HALELUYAH!!!! my engine was working great!!!! i went for a test drive, and i hadn´t felt the engine so smooth for more tan a year. So if you are still having some issues with your iddle, PLEAS CHECK YOUR VACUUM HOSES, SPECIALY THE LAST ON THE REAR, AND CLEAN THAT TINY PLASTIC DEVICE (stil don´t know its name) i hope this can help you too.