Originally Posted by
horsepuller
Since your main clue so far is that it acts like the key is shut off, I think that is the place to start with trouble shooting. Definitely sounds like loss of 12 volt power, intermittently. The lack of computer codes is consistent with this theory, as the computer system would see the power loss as a normal key-off.
I'd start by checking all fuses, and the main electrical connections at the battery and frame grounds. Visually inspect for corrosion, and wiggle for looseness. I agree with dal929 that a bad ignition switch could cause your symptoms; I've had a number of ignition switched go bad on different equipment. If you can get to the ignition switch you could try jumpering across the run contacts, after you start it, if it still quits, this test would rule out the ignition switch run contacts possibility. The challenge here is to devise tests to rule out components; as you say, you don't want to throw parts at it.
Good luck. Please let us know how your getting along with your trouble shooting.