High Idle Problem. Any Ideas?

[just_driver]

Hi. About one year ago, I got the problem with idle on my 1992 MX-3 (4-cyl SOHC): When vent ON, SOMETIMES the idle rises to about 1500-1600 rpms from normal 700-800 rpms with vent OFF (I'm talking about warm engine, of course). And starting about 1 month ago, with vent ON the idle ALWAYS rises to 1600 rpms from about 1100 rpms with vent OFF. If I turn my steering wheel, the rpms drop from 1600 to about 1200. Actually, the car runs pretty well, though it starts to produce some pulsing noise at about 75-80 mph. And (may be) the engine power is not as good as it was before (not sure)... Any ideas? Can I solve the problem with alternator replace? Thank you.