Just one idea here, may not be this, but I'm pretty sure it is based on the explanation.
if it's a constant wobble, low frequency, sort of like a constant hunt for center without ever stopping there,.. then it's most likely the integral gain in the PID loop for aileron. If that unit has only one set of PIDs for the swash, then you're going to have to lower the integral gain (I gain) for the entire swash(ail and elevator). If you go too low on I gain, you simply lose a bit of the consistency in rate (slightly less "heading hold" power). You want to find the point where the machine tracks in fast forward flight really well, and not go above that with the I gain or you can experience constant, low frequency, low amplitude wobble. the headspeed change does slightly change the requirement as it changes the overal speed of the control loop. When adjusting/tuning PIDs, always remember Proportional gain is overall amplitude of correction, Integral gain is for consistency (what many think of as heading hold), and Derivative gain affects acceleration (starts/stops). However, there are interactions among the three, and Proportional can be thought of as the "master" that controls how much overall correction is provided, with I and D tuning the characteristics of that correction.
i generally try to start with really really low I and D gains, zero if possible, and yes the machine will fly pretty strange. This allows tuning of P gain, raising then lowering till just below wobble, at all headspeeds. the machine will still feel strange and not fly well, but that's not the point, the point is to tune P gain. At that point I start raising I gain till it locks on a heading in fast forward flight. It's "there" when you can start a fast pass, 80 to 95 percent collective, on one extreme side of the flying field, at say 75, or 100 feet, and let go, and the machine ends up on the other extreme of the flying field exactly on the same heading. No need to go higher, as higher gain on I is not better. It should hold a heading and it's doing that. More gain can just introduce problems, as you're seeing (my guess as to your issue). At that point, then I do some sharp jabs of cyclic, normally in fast flight, but if you're not comfortable with that, you can simply do fast tumbles, watching for behaviour on stops. If it doesn't stop when i tell it to, I raise Derivative gain till it does. If it gois "boing" and springs back and forth before damping, than D gain needs lowered a bit. It should be high enough to get a crisp stop without rebound, and no higher.
If I and D are really difficult to get right, then lower P a few points and it will make it easier, as each adjustment on I and D becomes a bit finer.
Don't worry about making large changes _IF_ you are comfortable flying helicopters (intermediate to advanced level). I'm impatient, so I use sort of a "binary search" method. Say I have an initial setup, P is at 50, and first test says it needs more. Total adjustment range is 0 to 100. I go halfway between the setpoint (50) and max (100), so I'm at 75. If next test shows that is too much, then I go halfway between setpoint and previous setpoint (75~50), 62.5, so I round up, and set it to 63. this procedure can be a bit scary at first with the large changes, but it gets the setup optimized very quickly. Just remember with each change, fly very smoothly for a minute or so, then increase the aggressiveness incrementally. If the gain is way too high, or way too low, you want to fly in a way as to "sneak up" on learning that, but sneak up on it quickly
. Usually takes me 2, maybe 3 flights to get things really good, then it's minor tweaks, a couple points at a time every dozen on an intermittent basis,..maybe i'll notice something after 50 flights and tweak a touch, but the above procedure got me 99% "there" in a couple flights.
Sorry for the lengthy reply,.. but it's been a while since I posted a war-and-peace message and I was in a typing mood
