1986 Mariner 25hp HELP

[Spavinaw]

My motor just started acting up. It had been running great and out of no where when cruising across the lake and near top speed, which isnt very fast for a 25hp, it acts as if it was thrown into nuetral and the RPM's get real high. When I let off the throttle and the RPM's come down it goes back into gear and works fine until you gain your speed back.

My first thought was fuel. Can anyone out there help me out?

Thanks in advance for all the help!