1998 transport won't idle

My Van has been stalling when you coast up to a light or when parked it can't seem to control the idle. Now it did this last year too one time. I
fooled around with it then parked it. The next day it worked fine and did so untill last week. Now last year and this time I had been working on the van servicing it ie. air filter, oil change etc. So I don't know if I have a bad connection I'm disturbing or if its just a fluke.
When its cold in open loop its fine. But is seems once you go for a drive and warm it up it has troubles in closed loop. But of course not all them time!!! The first time it acted up last week, it wouldn't clear up but I only went for a short drive. I checked codes as the service engine light even came on and it had O2 codes so I cleared them.
I took it out the next morning and it was fine. The Next moring we took it for a 30 min. drive into the city parked and shut it off for 5 minutes. Started up and bingo it had its problems again. I had my code reader along so I checked codes. I got an O2 code again up stream sensor low voltage. So I drove (with difficulty!) 10 minutes to another place and went for lunch for an hr. Came out and it worked fine for the rest of the afternoon and we even shut it off a couple times!!! While in the city I picked up a new 02 sensor. As it seemed like when it was acting up I could smell the Cat. like it was running rich etc. So I figured maybe the O2 was faulty telling the computer it was lean when it really wasn't etc. I put in the new 02 today.
Tonight I took it into town and after 10 minutes it did the same darn thing!!! I brought it home and checked it over. Idle drops low untill it just quits. I pull the pcv out of the valve cover and that helped and kept it idling. But after a while like 10 minutes that didn't help either. It was late so I had to quit fooling with it.
So has anyone ran across this before on a Transport with the 3.4 ??? I don't have access to a scanner any more to check and see just what the sensors are reading. I'm thinking something is giving the PCM a bad reading or something. Also the darn thing is intermitant so hopefully it will act up in the morning!
Tonight when the van was hot and acting up I plugged in my old O2 sensor and grounded it to the motor etc. Started up and it didn't change a thing it still did the same thing wouldn't idle.
I don't know I'm thinking electrical as its intermittant something maybe with the O2 circuit or possibly Mass air flow, Map, EGR etc. Tomorrow I'm going to check the other sensors more closely.
Sorry for the long post!
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