Taking a wild guess here, as I am not familiar with that particular engine...
Try disconnecting the MAP (Manifold Absolute Pressure) sensor, and see if it runs the same. If so, it's likely the culprit.
When these go bad (or get intermittent) the engine can run at high RPM but will not run well (if at all) at idle and be very difficult to start once it stalls.
A leaking vacuum hose can cause the same symptoms (and not throw a fault light; you did not mention a 'check engine' light) as the MAP sensor will not get a proper reading. Spray the vacuum hoses and connections with WD-40 and note any change in idle speed. If these hoses have been cooking under the hood for 150K miles, they are likely cracked and rotten. The connections are likely not very tight any longer either. Every hose under the hood is going to die around the same time. If you radiator and/or heater hoses are changed, the vacuum hoses should be changed as well (same with the belts). Very often this is not done, and poor idle (at least) is the result.
No guarantees, but if it was my truck, this is where I'd start looking...
Owner: James Leach Global Expedition Services.
1995 110 Regular