Sounds exactly like what my '94 was doin', just about drove me nuts. Ran codes til I was fordblue in the face. Wound up being the vacuum attachment at the rear of the inlet plenum. Hard to believe that sucker would fall off because it's so hard to get on but mine did.
Starting at your PCV valve at the rear of the passenger valve cover, trace the line rearward and it changes to metal tubing. It then turns around the back towards the driverside and runs for about 5 inches or so past the EGR and then turns upward and has a short section of hose (a little less than 2 inches long) hooked on it. That hose connects to the rear of the plenum. This connection was the culprit on mine. The heat from the EGR valve can swell and just generally cause this small hose to deteriorate and it will literally fall off of the plenum connection. Pull the whole contraption out, replace that section of hose & reinstall. It's a booger to get back on because of the tubing inside of the hose but it can be done.
Also...while doin' all of this searching, be sure to look at your PCV connection on the valve cover. It's somewhat common for the little plastic elbow on top of the valve to crack/split. It's black in color and can be hard to spot down in there.
Another thing...There is a small branch fitting halfway along that metal tube mentioned above, check the small hose hooked to it there to make sure it's not leaking. It can suffer from the EGR heat also.
keep us posted on what you find,
'bird