The only things I can think of is that your area has a large amount of mineralized ground/black sand OR there is a problem with the machine. The 1280X has been around a while, it's what everyone used before the excals and CZ20.
My 1280X with the 8" coil works well here in NY in the salty wet sand/water however, you do need to be either in or out of the water for one thing. Shallow salt water running over the coil will drive you nuts with falsing and secondly, you need to turn the sensitivity down so it's not chatty, this machine can't be run "hot" aka sensitivity wide open it will definitely degrade the performance. I've turned the sensitivity down to as low as 5 in some spots and still dug good deep targets.
Good Luck,
TDNY