clunking noise

Hi
I have a 86 Yamaha 70hp,electric start with a problem.It ticks over perfectly,runs in reverse perfectly but as soon as it gets above 1000RPM in forward gear it starts to make a clunk noise every 5-6 seconds.Just 1 clunk then runs for a while then clunk again.
I disconnected the remotes & changed gear by hand and the prob remains so it's not the cables adjusted wrongly.
The timing appears to be spot on.Plugs are new
I've tried running it with each HT lead disconnected in turn.No change.
I've change the gearbox oil & there was nothing out of the ordinary with the old stuff.
The interval & sound of the clunk doesn't change with increased revs.
Any ideas???

[boats.net]

Maybe be jumping out of gear which means gearcase needs to be rebuilt.

I was kinda leaning towards that answer myself.I was also hoping that somebody might have come up with another answer.
A gearbox rebuild isn't in my top 10 of things I like to do