Thanks for your ideas. I've checked all mechanical and electrical couplings, there's nothing loose or dirty. I'm sure the fault is electrical but I can't think what it could be....
Since I'm only drawing about 20% of the current limit through the ESCs I don't see how they could be causing it... and why only on the water????
Yes Richard, it's both motors, I've had the top off and ran it against the bank so I can see what's happening, both motors are stuttering at the same time which is why I changed the receiver but I can't believe that two receivers are causing the same fault!
Still scatching my head,
Scottie