[CA] [DG11] ST60 Depth instrument

[Southerly 32]

I have a 10 year old ST60 instrument package including depth, wind, speed and smart pilot connected to a C80 plotter that has given excellent service. My depth unit has started to malfunction. On turning on the 3 dashes are displayed for 20 to 30 minutes. Then the depth reading given is very high and gradually reduces, jumping between different depths over a period of about 10 minutes until it finally settles down to give an acceptable reading. After this it will often display a number with the up or down arrows showing before resuming a normal recording. I have checked that the transducer head is clean. Can you tell me what the likely problem and solution is?