well when i first bought my 92 the whole dying after i started it up started to happen. i took to the dealer in which i bought it( the car was used 1yr old when i bought) and they told me that it was the EGR valve and that it was a warrenty replacement(of course that was about 9yrs ago) so as far as starting and dying the EGR worked for me. as far as the after 20min thing that didn't happen to me so i "might" be related. with as much as you have done in the last 4000 miles this should not to be too bad.
If you don't have one already, get a FSM. The greatest tool ever made!!!
First off check the idle control valve. tap on the housing with a screw driver and see if that doesn't help. Also check the idle screw. It should be most of the way in, if not all the way. Check the 2 prong coolant temp. sensor. These things always fail, and everyone forgets to check them! As cheap as they are ($8) you should always check them first.
Just for shits and giggles, check the MAF... those have been known to cause funny idling.
Look through the FSM, follow the trouble shooting section. I can't stress enough how helpful this book is!!!