The bouncing, cycling idle is one of annoying problems that doesn't affect performance but can drive you crazy.  The problems is typically associated with a bad or misadjusted TPS and / or intake vacuum leaks.  What's happening is the stock ECU thinks it has to accelerate the engine but then finds out it was a mistake.  This happens over and over, and this is why the idle speeds cycles up and down.

A misadjusted TPS can cause the engine to rev up and down, as the stock ECU is confused on whether the throttle is open or not.  Having the TPS adjusted to within spec will minimize the TPS causing this problem.
FC3S Pro - How To's, TPS Adjustment

Intake vacuum leaks can also confuse the stock ECU, which can trigger the BAC valve.  Please check out our intake vacuum leak page here:
FC3S Pro - Problems #1, Engine, Intake Vacuum Leaks

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