What you're describing sounds like the most glaring handling fault of all live-axle cars.

Any car with a live (solid) rear axle will exhibit this behavior to some extent. On a simple setup like the 240, it is more pronounced - on the more sophisticated 700/900 'Larsson' axle, it is more subtle.

The problem is that the rear wheels are rigidly connected to each other, and the axle is located laterally by a Panhard rod which swings in a slight arc through the suspension travel. The variable loading of the suspension through the bumpy curve (cornering and impact absorbtion combined) causes lateral jiggles in the body. Then the impact experienced by the bumped rear wheel perturbs the remaining rear wheel, possibly to the extent of breaking traction momentarily. This is what you feel when the car sidesteps in a bumpy corner.

The solution is a multilink rear suspension, but that's not really feasible for the 240. Your best bet is a good set of shocks (Boge Turbogas should work well, if not try Bilstein) and possibly a set of progressive springs.

The 240 is pretty well-sorted for a live axle car - try that same corner in an 80's-era Mustang and see how terrifying it CAN be!