96 Ford Explorer rough running

96 Explorer, 6 cyl, with intermittent rough performance, and occasional difficulty to start. I recently replaced MAF, coil pack, spark plugs and wires, fuel pump and filter. I removed and inspected fuel injectors, and the cylinders compression are within limits. I almost purchased a camshaft position sensor, but employees at auto parts store did not think it would help. I have been getting engine code 0306 mostly with occasional 0303 and 0305 codes (misfire cyl 6, 3 5 respectively). It seems to run ok at cruise, and at idle when warmed up. While accelerating, I get code which corresponds to enriched fuel mixture. Cold mornings (40-45 deg in Sacramento, CA) may either be very difficult to start, or may start easily with RPM fluctuations until it warms up in about 1 minute.