Problem #1 can have many causes. I don’t know what ideas you have tried from Humminbird but let’s see about these:
When this happens, is there another sonar unit on? If so, try turning it off and see if the problem clears up or not. If it does, you can try turning it back on and turn the Noise Filter menu on the 788c on or up to a higher setting.
Is there anything else running on the boat? If so, turn it off as this could be causing some sort of interference.
What water depth does this happen in?
What software version are you running on your 788c unit?
Which 788c model unit do you have? We have made a 788c, a 788ci, a 788ci HD and the 788ci HD DI model.
Try and find someone else that has a good working Humminbird unit so that they can test your unit on their boat. This does not have to be another 788c unit as most use the same 200kHz 2D sonar. This can be one of the Side Imaging units but not one of the newer DI model units.
For problem #2 you can either change the mounting hardware, install the third hold-down screw in the mount you have or relocate the transducer to an area with less water force against the transducer.
For problem #3 you may want to consider running a new set of wires all the way from the unit directly back to the cranking battery. 16 gauge multi-stranded copper wire, an in-line fuse holder, 3 Amp fuse and connectors to the battery itself will help ensure a good electrical supply for the unit. Use all new stuff as anything used is suspect and may not work. Solder and seal all electrical connections.
For problem #4 you can set the Chart Orientation menu to the North-Up setting. When moving slowly, the GPS cannot reliably determine the direction of the boat. So when on the Course-Up menu setting it will default back to pointing north when it cannot determine direction.