My '86 120 looper had this problem last year. Over the winter I re-built the 4 carbs and did a link & sync but the problem is still there. Starts fine, idles smooth and runs great at WOT 
but if I cruise at 3500 RPM it will run fine for a minute or two and then drop about 500 RPM for about 6 seconds and the pick up again for about 10 seconds and then drop again repeating this cylce . If I increase to 4000 RPM the cylcle goes away and it runs fine. If I slow down to say 3000 RPM it is fine too. Anyone any idea what could cause this?
but if I cruise at 3500 RPM it will run fine for a minute or two and then drop about 500 RPM for about 6 seconds and the pick up again for about 10 seconds and then drop again repeating this cylce . If I increase to 4000 RPM the cylcle goes away and it runs fine. If I slow down to say 3000 RPM it is fine too. Anyone any idea what could cause this?