We had a little excitement last night around 10pm, NEXUS was on autopilot as usual, and started veering back and forth slowly between port and starboard as if a drunken sailor was at her wheel. I checked Laurie's breath quite carefully, and this turned out not to be the case. The weaving course was exacerbated by the fact that we were flying our heavy Gennaker in 25kts of wind, so NEXUS took off like a Ferrari when we veered windward to starboard and calmed down as we veered leeward to port. I hand steered her for a few minutes, and found it difficult in the wind and following seas, but not impossible, and we've certainly been in more challenging conditions before with no problems. So with all hands on deck, we pulled the Gennaker down and started the motor to do some cautious trouble shooting, and she wouldn't even hold her course under motor. I methodically checked every component of the autopilot, and steering systems, checked for metal next to the fluxgate compass, nothing was wrong. I switched to autopilot to Navigate mode, and NEXUS bounced from curb to curb down the lane on the chart plotter, carving a sinusoidal curve in her tracks. I stared at the instruments as she slowly weaved back and forth. We have multiple instruments at the helm indicating true and magnetic course, and multiple redundant sensors capable of providing data for backup purposes. I noticed the readings of the true heading from the GPS, and the helm compass were consistent with each other and differed only by the charted magnetic variation we would expect, however the magnetic heading readout on the Autopilot was off by varying amounts up to 30 degrees and literally wandering back and forth inconsistently. I did have a drunken sailor on board, it was my Fluxgate compass! I checked and it showed three green lights as usual, and there was no evidence of Captain Morgan spilled around it, but it sure as hell was drunk. So I switched the Autopilot to the backup satellite compass installed on the arch, which is GPS based, and all returned to normal. Over a cup of fresh coffee, I thought, "well isn't that special!" I've never heard of a fluxgate compass going through this kind of failure mode, and I look forward to the look the face of my electronics guy back home. I have a spare fluxgate compass on board, but you have to spin the boat three times in calm water to set the deviation table when you install it, so since we still have an additional GPS heading sensor backup installed, I think I will wait until we get home to swap it out. You can't make this stuff up! We are currently running up the west side of the Turks and Caicos Islands, and we are watching a low pressure cold front that the GRIB files predict will cross in front of us off of Cape Hatteras just before our predicted arrival time there. If it holds true, it would cause wind against current in the Gulfstream, which is not a good situation around Cape Hatteras. We'll make a decision later today or early tomorrow in consultation with our weather guy, and we'll either head north to home, or west to the Bahamas. Everyone is fine on board and in good spirits, so all is well on NEXUS.