the hds unit used to be on the front of the boat and I had an elite 7 chirp at the console. The hds had the spotlight scan ducer. I was fishing on fork and it was like an ocean and my trolling motor came straight down on a stump and broke the ducer. So I flipped the hds to the console and bought an lss2! And bought the bracket for the hdi skimmer to mount on my trolling motor! So now the 7 chirp is up front and hds is at console! And yes I do have the lowrance map sd card in there! The side and down scan both freeze in a perfect rhythm they will scroll for 2 to 3 seconds then pause for 2 to 3 seconds! Hopefully I cleared up what has happened a little better!
So you are telling us the HDS unit used to be at the bow. Earlier you told us it was an HDS 7 Gen 3. I'm curious how long it was operating at the bow - since you say you have a Gen 3 unit which was released for sale only recently. Did it ever have any trouble there? The Lowrance connectors can sometimes be a bit tricky. The sonar will stop if it can't detect a transducer. I suggest you unplug the LSS2 cable, rotate the locking ring completely to the rear of the connector housing, fully seat it straight-into the HDS, then insure you fully engage the locking ring. I'm wondering if you really don't have it plugged in well. I do not believe that adding an (optional) 2D transducer will change anything.
If your HDS 7 is a Gen 3, it's practically brand new. Have you called Lowrance Support?