1992 Cougar high idle

Hi. I've had a '92 Mercury Cougar for a few months (145,000 mi), and it always seemed to idle a bit high. Last week it seemed to get much worse, and it now idles very high, at 2500 RPM in Park and 1200 RPM in Drive with brakes on. I can drive 35-40 mph without giving gas. My mechanic checked thoroughly for vacuum leaks (lowered rpm for testing in some nifty way and carefully checked everywhere, including removing to intake manifold to check the gasket. No leak found. Sensors all test OK. Codes not showing anything (though Check Engine has come on then off a few times). The injectors, however, are showing very high voltage, so he thought brain box must be faulty. I got another for $30 from the junkyard: no change. Of course it's possible that the "new" one is bad in the exact same way... I don't want to drive it much, because it's bad on gas, it's stressing my brakes out and I heard it could blow a head gasket, never a strong point on these cars (Ford 3.8 V6) anyway. Can anyone help? Thanks.