" I have a 1992 120hp Johnson which will be running along on plane just fine, then all of a sudden...THUD. It shakes the entire boat. At first you think you have struck an underwater log with the lower unit and you can physically see the motor quiver. However, there is no loss of power or motor performance. The motor& boat speed does not change at all. At first, this condition started off very infrequently now over time the frequency has increased. Now, its a very regular condition. It seems to happen every so many minutes while running +60% full throttel.
Would appreciate any input on the cause/fix for this mysterious condition.