haywire

After the recent fud fest II watersking weekend, my speedo has going haywire. IT works for about 500 yard, then flicks up and down some random speeds, before just showing 0mph all the time. Any ideas what this might be? Could it be water etc, if so why does it work for a wee while before...

Noticed my speedo going nuts and all over the place a few weeks ago so I bought a new speedo sensor to solve the problem. It will be the 3rd speedo sensor I've had to replace on this bike. However, fitted the new sensor at the weekend and went out for the first time since Saturday on the bike...