Have you checked the vacuum lines and connections, I have a MityVac for that IIRC was about 40 bucks or so. I ended up replacing several connctors (rubber vacuum line from OReilly's) and one of the hard plastic lines, again rubber line.I just hope its none of the real pricey stuff, like the EHA($280), Idle control valve($150) or the Fuel regulator valve($250).
The EHA, regulator valve and others can be gotten at a you-pull-it yard, they are specific for the 6's but you'd have more to choose from as long as it's the M103 engine (300TE, 300E, as well as the 300SE/SEL). It's a crapshoot for the EHA but I found one that was still holding gas under pressure, no leaks. Also the timing controller on the left fender, can't remember it's name just now