I think this problem has come up before, from memory it was to do with a voltage drop when the engine is starting and that causes the electronics to shut off coz they are below their voltage working range, or something along those lines. Hopefully someone can elaborate and let you know a solution 

 loose earth to the souder

 Id be checking for corrosion on your battery isolator, they are almost always mounted in poor positions 

 All connections must be clean, dry and tight. And check the wire itself--if moisture gets inside the insulation via a nick or just bad installation, it creeps along unseen increasing resistance and will give you a lot of voltage drop. Paul above is onto it--it will be to do with voltage drop. Your other unit doesn't do it, so that narrows it down a bit as to where you need to look.

Thanks for that fellas, that sounds spot on. Looks like a couple of hours cleaning and rewiring when I get home.
Much appreciated.

it will be volt drop caused by the cable from your battery at the rear of the boat through to the helm is to small to run all of the equipment , when you crank your motors the high current draw drops the voltage and the sounder senses lower voltage and shuts down, bit can be fixed by installing a by installing a bigger cable ,    Also do check all battery and lead connections to ensure good connections as well 

 Yeah mate, it's usually caused by inferior batteries in terms of cranking amps available which means voltage in the circuit drops below say 10 volts or too small a cable from isolator to distribution panel, I run an HDS 12 and HDS 5 I had the same problem, I ran a 6b&s cable from the isolator and negative. Fitting a better battery would help tooo, is suggest a good Delkor battery. Give me a yell if you want to have a chat about it 

Cheers for that fellas.

 I have had the same problem in the past and it is do with voltage drop. Your cheap sounder may not draw as much current as a better sounder therefore it stays on while the other one goes off. My voltage drop was caused by a faulty winding on the armature of the starter motor of outboard. Didn't cause any problems starting the outboard but did cause the voltage drop which made sounder turn off.

but Lowrance can be wired for auto on every time the key is turned. No idea why anyone would want this but its the yellow wire I think (disconnect it from power)

Just an update, after spending all day going over the electrics I finally narrowed it down to the positive battery lead terminal crimp, hard to find as there is thermo shrink over the crimp to the terminal but it feels a bit loose. Ran a temp lead from the batt to the isolator switch and everything works. The guys at chivers are going to make up a set of heavy duty leads for me, also purchased a new lowrance hook 9 while I was there. usual great service and price.

Cheers for all the advice.