Maybe its something as simple as raising the idle speed? As you said, 500rpm is too low. It could be just too low and the engine is trying hard to keep it from stalling.

Injectors are definitely suspect though...I had a real bad rhythmic surging problem, but no real power loss. After I replaced my injectors, the idle was still rough, but this rhythmic surging stopped. CIS can be a picky thing.

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