I don't think that the drop in idle is caused by the alternator - more like your intake needs cleaning.
With a fully charged battery, you should see 12.6v @ the terminals.
With the engine running, you should see 13.0v - 14.5v @ +/- 1500rpm with all accessories off @ the battery terminals.
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2001 Boxster S (triple black). Sleeping easier with LN Engineering/Flat 6 IMS upgrade, low temp thermostat & underspeed pulley.
2001 MV Agusta F4.
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