Rick,
As noted, the reason for the instability is that the longitudinal axis of the tank (and combined weight "above" you) is high enough above the longitudinal axis of your body that when you roll laterally that weight wants to continue rolling laterally.
Best thing to do is reduce the height of the (longitudinal axis of the) weight above you. Best way to do that, initially, is to reduce weight period - to an extent that is safe.
Next thing is to improve lateral stability by re-locating weight - including bottles. On the bottles. One of the better ways to carry argon is with a 6 cu ft tank attached to the backplate along the left side. UP has a series of photos of a mount he made. It works well, and doesn't cost much. If you want one that works well and costs a whole bunch, check this one out:
http://www.halcyon.net/exposure/accessories01.shtml
Halfway down, the 6cuft item: photo is not good, but the two small bands bolt to the side of the bp.
If you're going to carry a bailout bottle, one of the best ways to do so is slung as a deco bottle. This allows you to ensure that no gas loss will occur (by turning off the valve), but that you can turn the tank on and use it rapidly as the valve is in front of you. Also, if hung up or entangled, you can more easily remove or untangle a situation that you can get at and see. Additionally, this puts the weight of the valve and regulator down beneath you instead of to the side off your single - which adds to lateral instability.
FWIW. YMMV.