2006 yamaha 70 problems

I am having trouble diagnosing a problem with my 70. Motor will run 30 minutes or so at 4200-5000 rpms and then drop to 1200rpms, will clear itself up, sometimes it takes 4-6 minutes or sometimes 3 hours. Checked the coils and the 2nd cylinder was not firing so I thought I had discovered the problem but motor will jump back up and run for a few minutes or sometimes all day. I changed plugs while motor was acting up and no fire in 2. Mechanic says the coil can't work,not work,work,etc.. has to be the carbs. He cleaned the carbs on Friday and Monday I had the same problem after a 30 minute ride. I did not check the coil on Monday and the problem lasted 3 or 4 miutes and then not again. Thanks for any help. Pat

[boats.net]

Check the warning buzzer, if not working it may be going into safe mode and you wouldn't know. Check pulsor and charge coils output voltage with DVA meter at time it's loosing rpm's.