Running FM1000 and Trimble WM-Drain, I'm getting a spike in VDOP at random times. Been happening for awhile and some days/ places are worse than others. The spike will happen, kick out automatics, and then drop back to the 1.2-1.3 range right away. Is this an antenna/ coax issue? The AG25 I'm using was new 2 years ago, the whip and other cables are older.