The RPM in my 95 Sportage engine will quickly drop and surge at highway speeds. Around 55 to 60 mph, the RPM will fall and rise 250 to 500 rpm below and above the normal cruising RPM of about 3000 without any additional input to the throttle. Under acceleration, the engine pulls smooth and steady, and around town this problem doesn't exist. Anyone have any idea what may be causing this? The Sportage has about 290,000 mi., is an automatic, and is bone stock. Thanks for any help.