I had a similar problem: 1.6K & GEMS ECU: persistant misfire at 6000 rpm in 5th & 6th. Mapping was OK as was the alternater, battery, leads, coil, dis. cap, rotor arm, plug gap as all were checked/changed! It was completely cured by changing from Bosch FR7DC(new plugs) to NGK BCP7ES plugs.
Mick