I'm with fisherlady. If it clears up fairly quickly after he slows down and works fine at trolling speed, I'd say a propwash/bubble issue. What size/type boat does he have and where is the transducer mounted on it?
http://www.lowrance.com/Support/Tips-and-Tutorials/Transducer-Guide/ "Most of our permanent-mount transducers are designed for high-speed operations. For the best results, the transducer should be placed where a smooth, undisturbed flow of water will pass across the face of the transducer at all boat speeds. Read your transducer's
owner's manual before installing the transducer!
<< Transom Mount
The transom-mount transducer is the most popular, and it's generally the easiest to install. The Skimmer® transducer design performs best when it is slightly below the boat's hull. A plastic transducer is recommended on aluminum or steel-hulled boats to avoid potential electrolysis problems.
Mounting the transducer on the transom is recommended for outboard and stern-drive (I/O) powered boats only. Transom mounting is ideal for high-speed operation and models with the "kick-up" feature will prevent damage if the transducer strikes an object.
Make certain that the chosen location doesn't interfere with the boat's trailer. DO NOT mount the transducer directly behind the ribs, or thru-hull fittings. Typically, on aluminum boats, mounting the transducer between two ribs works best. On all hulls, mount the transducer at least one foot away from the engine's lower unit. This helps to prevent air bubbles from the transducer interfering with the propeller.
Periodically wash the bottom of the transducer with soap and water to remove any oil film or growth that may collect. Oil and dirt reduce the transducer's sensitivity and can even prevent its operation."