Does it feel like you hit a log with the lower part of the motor? Check your lower unit oil. Check for metal shavings. Had the same thng happen to my Johnson. Would be going down the lake and all of a sudden the motor would act like it hit a submerged log or rock and lift up. Could go another week or so and nothing happen, than same thing happen again. The dog gear finally went and lower unit seized up. Had to replace lower gears.