of the transducer. You will have to move it to a different spot that gets the best reading.

Goodluck

 Probably need a bit more info champ, make, model, of your sounder and transducer, photos of your boat/ transducer placement, how long it's been happening.

 Not auto will help

As sandbar said it's probably your transducer placement. Usually caused by air getting underneath the tansducer. The tranducer could either be too high or it could be behind a strake on your hull which is causing turbulence. The transducer should be sitting flush with the bottom of the transom, or a little bit lower.

 Cheers lads thanks for your help,

I'll let u know how I get on 

i have  raymarine dragonfly sounder on a trophy 2002

it works within about 6 meters of water then once i either go deeper or faster it cuts out?

then when i slow down it doesnt work again?

it sucks :( 

would put my money on the transducer being positioned incorrectly.  Even if the unit is clear of the rear strakes, check that there arn't any forward lifting strakes that finish well before the the stern, but are in line with the transducer.  Also check that the transducer isn't too close to the prop (also what side is the unit mounted??)