86 1/2 nissan hardbody pickup idle?

Does anybody have any idea why my truck would have a rough idle some of the time. Its not a constant problem, its very sparatic. It will run great,
then out of the blue just start running rough. It idles fine, then the engine races, then drops to a rough idle, then races again. It stays in this pattern for a while, then quits and works just fine.
Why? I replaced all of the vacumn lines, no luck
any ideas?? Chad
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