I was kind of waiting till the more experienced gurus jumped in, but...they may be asleep. Shhhhhh...
Your problem could be almost anything at this point. Basics: Check plugs and plug wires. A piece of carbon on the electrode of a plug or a plug wire breaking down. Check the diagnostics for trouble codes (see your service manual for correct methods to pull trouble codes from ECM. Even though you say it disappears at higher rpm, it could also mask itself at higher rpm (i.e., less noticeable). Check the coil. A couple of simple tests can usually tell you if the coil is fading out. (Don't foorget to check coil to distributor wire). An ohmeter lead at each end of a plug wire and few wiggles will tell you if the wires are okay. EGR valve is also a candidate and easy to check. Doesn't really sound like a vacuum leak problem, but look for a disconnected or loose harness in the engine bay. There are several dozen more things to check. Stick with the basics, though. It takes fire, fuel, and air to make a motor run. Take them one at a time. The second "sticky" thread that is posted in this forum has links to a service manual that someone posted online. It has some troubleshooting and diagnostic instructions and is easy to download or read online. Hope this helps. I have an FSM, but it stops at 1988. It does have a great deal on 22RE engines, and that's what I have in my truck. All else fails, email me and I will try to be more specific in helping. (fa22trainer@yahoo.com)