My 4x4 does this all the time, there are 3 multiplugs at the back of the engine fixed to the bulkhead, it may be what Tab has said, or bad connection in one of them, i'm sure it's the middle one, but check all 3, these ALWAYS fuck up and alot of people solder the wires toghether and bin the connectors all together.
It can also cause a missfire as the ecu doesn't read the correct RPM.