...I know that the ladder line the G5RV is part of the antenna...
Actually it is part of the matching system
- it isn't intended to radiate.
Could you watch the SWR vary on a meter? If so, did it tend to move around smoothly, or did
it jump up and down?
The impedance seen by the tuner will change if the matching section gets close to metal or
other wires. So if you could see the SWR varying up and down, sometimes passing beyond
2 : 1 (with the antenna tuned), then it could be the movement of the matching section (or
the wires) as they sway in the wind, etc. Did you bring the coax for the other antenna down
parallel to the twinlead? Changing the spacing between them may change the SWR.
If the SWR changes are more sudden, that would tend to indicate an intermittent connection:
solid conductor wire is prone to fatigue and breaking at joints or bends due to flexing (as is
stranded wire where solder has wicked into the strands.) Intermittent connections at the
feedpoint and/or the junction between the coax and the twinlead are not uncommon. It is also
possible to get a break in the wire inside
the insulation if it has been kinked, a poor joint
on a balun, shorts to the mast or the other antenna, etc.
The fact that the problem got worse when the wind increased would tend to suggest one of
these scenarios, though there certainly are other possibilities. You'll have to apply your
knowledge of how the antennas were supported, their history, etc. to judge which is
more likely. Certainly if the SWR was jumping rather than changing smoothly a close
inspection of the antenna would be in order.
You can test your tuner into a dummy load or your normal base station antenna to see if
it shows any signs of problems, but unless the tuner was arcing or has an intermittent
connection (which certainly is possible) it sounds to more more like a problem with movement
of the antenna and/or feedline, especially if you just tuned the antenna once and didn't
readjust the tuner while you were operating.