roadsend,

"losing bottom" can mean a few things, but basically, it means the finder gives false or nonsensical readings...

You could be fishing in 120ft of water, but the depth numbers on your unit might start flashing, which means it can't figure out how deep the water is, or your unit might give you a false reading, like tell you you're in 6ft of water or 600ft of water, when you're really still in 120ft of water.

There are many different things that can cause this, here's a few:
Physical interference - everything from air bubbles and cavitation near your transducer, to kelp or something on the transducer, to your skimming transducer got whacked by something and is now pointing back behind the boat instead of down. Lots of these physical problems are caused by installing the transducer in a bad spot.

Electrical interference - everything from fishfinders on other boats near you (generating the same mHz rate as your finder), to interference within your own boat, to transducer cables running too close to radio and radar cables, "noisy" power cables from your motors' charging systems, bad grounds, etc. You could also do your unit harm by having it on when you crank your motor(s) over... which causes a surge of power that can damage its eletronics.

Software/cheap unit problems - sometimes the units just aren't that good, or the internal software that runs the units aren't great... which causes errors (think "crashes") also.

I think every company has their ups and downs, and what we're seeing these days is a lot of companies in transition, moving production overseas, or moving it from one foreign country to another... and every time they do this, they are sure to have problems getting the production standards back up to par. I think this is why we hear that Joe had great luck with unit Y, but John bought a new unit Y, and had terrible luck. IMHO, it's all just a shake-down of this move to produce units overseas to bring cost down.

I think one should just buy the best unit that they can afford, from someone they trust, and make sure it has a warantee that they're comfortable with, in case they need to return it.

-N.