Ford Repair: 1997 Ford F-150, VIN 1FTDX0864VKA85619, vacuum leaks, crankshaft sensor

Question
Problem with this XLT, 4x4, 4.6L, 120,000 miles pick up truck is that a few minutes after start it can't keep stable RPMs which suddenly drop from nearly 2000 to 400 and keep going up and down until the vehicle turns off. The mechanic suggests the crankshaft sensor may be faulty, but I'm in doubt.  Advice appreciated.

Answer
Hello,
   First, verify that there are no intake air leaks before the throttle body, and no vacuum leaks in spots like the PCV hoses. If there are any diagnostic trouble codes in memory(check engine light on or has been on), that might give you a direction to look in. You also could have a faulty PCM, which is more serious. I don't really see too much crank. sensor failures on those, and it's behind the A/C compressor, so thats alot of work for a guess.
    I hope this info helps, and I'm sorry for not being able to get back to you sooner. Good Luck!   Shaun.