service engine light goes on at 70 mph and stays on.

The computer indicates multiple misfires. I have changed the plugs & wires. Service techs recommend a valve job but there is only 66K miles. Another wants to replace the engine coil or the fuel filter. Did anyone out there have a similar problem? The light is now on. The light blinks and the engine runs roughly at times - usually under stress. The light is now on all the time.