High and Low RPM at idle

My 93 euro 3.1 has developed a strange sporatic idle problem - after driving at a steady speed for at least 5 - 10 minutes when I come to a stop the engine seems like it is going to stall, the idle drops low and then goes high about 2500 rpm and then back down and up down and up until I put it in neutral then it seems to level off at a normal idle but goes back to the problem when I put it back in gear. This problem does not happen all the time. It got real bad and the engine stalled and would not start. I towed it to a shop where they told me the fuel pressure was only 10 lbs. and this was the cause. Well, the fuel pump was replace for $400 + and was fine for a very short while. The problem has returned. Any ideas on where to start would be a great help.