Hard to tell from the picture, but looks like its mounted to low.

Try switching it to the other side of motor and possibly looks a little low...

 A few more pics from the correct angles would help but looks like it might be mounted too low, too close to a planing strake and maybe better on stb side as they often say its better mounted on the side where prop blade is travelling down to avoid pressure wave.

Have you leaned over the back for look while someone else is driving?

Have you downloaded the transducer installation manual?

Hard to tell from the angle of the pic but it looks a little too low.  When looking from the rear, the centre (up/down) of the right side of the transducer should be level with the hull. 

That being said being too low doesn't sound like your problem.  Your problem sounds like air bubbles are getting under it when you get to speed.  If this is the case when it starts to break up you'll get vertical lines across the screen and missing chuncks of the bottom until losing it all together. 

First check the hull is smooth in front of the transducer and not causing turbulence. 

Then get a mate to drive next time your out - lean over the back and check it at slow speeds when reading well then get him to increase speed until it breaks up.  You'll probably notice the water around the trans starts to be filled with air bubbles/turbulence.  see if anywhere else at the rear is clear (should see more blue or green water coming out from the hull without the bubbles) and this will be where you want to mount it.  You may find it's too close to the strake on the right and air/turbulence is coming off this moving it an inch to the left may help but hang out the back for a looksee first before drilling new holes.  Good luck

Cheers Paul

 Thanks heaps guys, Unfortunatly the boat is fitted with a pod so hanging over the back isnt an option I was thinking of moving it more to the centre of the strakes. As for height I have tried different heights and angles all with no success! I have updated the actual sounder and gone throught the manual like a man posessed to fix it.

put a mirror on a piece of conduit and hold it over the back to see just what sort of turbulance the transducer is trying to operate in, just a thought

Try to alter the angle  of it so that it will be more nose up bum down, that should stop some if not all the turbulence occurring underneath it before drilling new holes.

It looks to be too close to the edge of the strake that is possible the cause of your trouble. Moving it a couple of inches to the left will solve everything if you decide on anything.

Too low and too close to the strake ;)

 Thanks all I will remount today and do some trials, thanks to all for all your comments and input.

Try going left a touch

Guys at Chivers (where purchased) were very helpful but it even stumped them. I eventually replaced the tranny (it was only 6 months old) and all ok. The other thing that I tried before the new tranny was change the contrast colours. I found it worked better with brown bottom (standard colours). Only God knows why though.

I'd definitely try the heel down a bit more though and move outboard. As for the distance from the prop - at your distance it wouldn't make much difference what side of the boat the tranny is fitted.