I sort of agree with some of what Haiko suggests, but I think you may be confusing some harness line issues with arm strain issues.
Did you just start wearing more of your winter gear recently? Are you wearing gloves yet?
Before I launch into why changing to thicker winter wetsuits and sailing in gloves tires your arms out much more rapidly than when you sail in board shorts and a rash guard, please tell me if you recently made this change to thicker warmer gear?
Also there is the phenomenon of sailing powered up to overpowered, vs sailing less than fully powered up.
When you get fully powered and start going quite a bit faster, there's a tendency (with pretty much all sailors I think) to begin to grip the boom much more tightly, not lean back with your hips to take the pull off your arms, and to begin to take over some portion of the load on the boom directly with your arms (meaning you begin to bend your arms and pull rather than keeping your arms fully extended and loose, so you can "Play the Piano" with your fingers on the boom).
If you are indeed "oversheeting" when you sail into a lull (whether due to a natural tendency to "fall back" away from the rig and bringing the rig with you, or just because it feels like you are falling into the water backwards so you pull the rig in in an attempt to pull your self up over the board).
Either way, oversheeting the sail kills it's power and actually makes the situation a bit worse as you are decreasing the rig's ability to hold up your body weight.
What to do about this...... corrective action so to speak.....?
Train yourself to look ahead an a little upwind to see what's coming your way (a gust...
or a lull).
If it's a gust, head a little higher, sheet in a little more and use the extra power to get upwind, or.... stay on the same course and don't sheet in much allowing your board speed to increase significantly.
If you see a lull coming, head off a little, sheet out a little and start bringing your weight in over the board BEFORE the lull reaches you. In other words, look ahead and upwind, and anticipate gusts/lulls while you have the time to respond without falling back or falling off plane, or getting jerked over the handlebars.
Hope this helps,