Originally Posted by Susie's Suby
It runs strong over 4000 rpm's, but wants to die below, while you are driving. It will idle about 10-15 seconds smoothly, the drop to below 500 rpms and shake. If you give it the gas it will pick up revs immediately, but then repeat the low idling and shaking. It is very frustrating.
Could be a vacuum leak. If you can, try to listen under the hood while the engine is running -- a vacuum leak can often be heard as a loud hissing noise. An engine with a vacuum leak (cracked rubber hose, usually) will show exactly the symptoms you describe: smooth and normal at high revs, but stumbles and dies at idle.
You could also check the idle air control valve (IACV or IAV, depending on who you talk to). However, a vacuum leak is the first thing I'd look for and it's easy/cheap to fix.