DSM300 losing signal

[Tolly guy]

I have a E120 connected to a DSM300 by way of a Seatalk HS (cat-5)
cable with a crossover. My transducer is a Airmar Triducer B744V. (see photo)
It will work for a short time then alarm will say "DSM connection lost"
When I try to set the model in the fishfinder set up mode some times it will not highlite the B744V or just revert back to an other...PT??

I recently got a second DSM300 and and a RL80c.
I swapped the DSM300 but no change.
I have the RL80c running through a HSB2 cable, which was working, then I plugged in the Seatalk cable from my E120 and lost the signal to the RL80c. But when I loose single "DSM connection lost" both displays will alarm. Could it be my transducer? Bad cable or cross over?

My questions are:

A. Will one DSM work on both E120 with ST highspeed cable and the RL80c with the 3 wire HSB2 cable.

B. Should I run my other DSM300 to my RL80c and get an other transducer, That way I will have two stand alone systems.

C. Which transducer would I use for the RL80c with a separate DSM300.

Thanks in advance