Every unit I have had will lose the bottom on occasion, especially in deeper water ie, 100 ft. or more, or when going at faster speed, which changes angle of transducer.
Have you tried adjusting angle of the transducer slightly one way or the other ?
Always possible you have transducer problem but try the angle adjustment first.
I have had Lowrance units for years, never had a problem, and they track the downrigger ball on screen, so that's shows they work fine. I found them alot better than a few of the Humminbirds I owned.

Sounds to me like its the transducer. And may also need to send the unit in for the up dates. I had the same problems your discribing. And replacing the transducer and the updates did the tricks. I think it will be around $ 200.00. To get it working right again.

My brother and a freind have both had this same problem.Angle of transducer was fine in both units and the problem turned out to be the tranducer both times.Both units were also in the same 5-6 year old range.Note,I also pick up the thermocline,downriggers and bait pods on my low end unit(500 doller) polebreaker