Just when I think I understand this engine. I've got a high mileage truck ('89) that I tuned up about a month ago. It was running great... for a week. Good acceleration, great mileage (28 MPG hiway). Then the idle speed began to decline followed by hesitation on acceleration followed by the check engine light flashing on. This is the list of the things I've done:

Replaced cap, rotor, wires, plugs
Removed and cleaned throttle body
Replaced pcv valve
Replaced O2 sensor
Checked and cleaned EGR circuit
Reset Throttle Position Sensor
Adjusted Valves
Set timing
Replaced suspect vacuum lines

Net result: Still hesitation, still check engine light, still great mileage

The check engine code only flashes on the highway and will go off instantly when I release the throttle pedal or if I floor it, and stay off for another ten miles or so. The code indicates "O2 sensor" but seeing as it's new I think it's just registering excess or not enough oxygen in the exhaust stream. I've adjusted the idle screw all the way out and can barely muster 800rpm at idle. There's no fast idle on a cold start. If I don't tap the throttle a couple of times it just dies. Once it's warmed up it doesn't die but idles too low. To me this points to an air leak on the intake side, but I've checked and rechecked all the vacuum lines and did the flammable liquid thing on the throttle body chamber (there's got to be a less dangerous way to check gaskets) with no change in idle speed. I'm stumped. Any suggestions?
Is there a step-by-step way I can bypass elements of the vacuum circuit to try to narrow down where the problem is, or am I on the wrong track?