I think what you are saying is that as the front track gets narrower, you become totally dependent on the rear springs for roll stiffness. Any side force can only be resisted by the rear springs. To get roll stiffness you need either to have stiff springs in the rear (so small deflections cause a lot of change in spring force) or a rear anti-roll bar (sway bar) that is twisted as the body rolls side to side.
If your suspension has any roll steer, a side gust could cause it to slightly turn each time the body rolls, which could get you into a side to side oscillation. As mentioned king pin angles and scrub radius etc. can also cause steering effects as the body rolls.
You need to set up some tests where you can measure what your car does as the body rolls (at its ride height at speed!!). For example place dial indicators on the front and rear of one of the front wheels and then slightly roll the body and see if the front wheels are turning slightly. As mentioned many simple measurements can be done with just some mason twine and plumb bobs to check how your tow in and tow out changes with body roll and if your rear axle turns slightly as the body rolls.
Things as simple as soft suspension bushings could be the culprit.
Take some measurements of all your suspension links and draw a scale diagram looking at the suspension from the side and see if raising or lowering a rear wheel (ie body roll) causes the suspension links to move the rear axle hub forward or backward as the wheel rises or drops. Depending on how your front steering is set up your front wheels could be changing toe-in and toe-out as they hit bumps this can also cause steering effects.
If your front wheels are toeing out at speed that can also cause "darting" where the car gets unstable in direction.
http://www.ozebiz.com.au/racetech/theory/align.htmlIf you can find someone that has this software it will do a lot of calculations for you once you make the measurements and plot curves for changes in toe and that sort of thing.
http://performancetrends.com/SuspAnzr.htmAsk around the local circle track racers and find out who the local suspension setup guru is. They spend a lot of time learning about how changes in suspension link locations and lengths changes a cars handling. It might be worth a six pack to invite the guy over to look over your suspension to see if he can see any obvious issues that could lead to directional instability.
Remember at speed if you are getting any front end lift, your suspension is not in your static parked position at speed and might be doing something completely different than it does when the car is parked in the garage.
Larry