runs ok then rough

[6horse]

My older 6hp (i believe 1975) starts fairly easy and runs ok for about five minutes then runs rough for a few minutes then ok for a few. It does this continuosly, ok then rough then ok then rough. I've changed the spark plugs and ran "fresh" fuel but no improvement. The wires look good. I'm not sure what to check next. I'm considering the carb. Any input would be appreciated.

[boats.net]

May need carb cleaned, may be misfiring on one cylinder.