Weak springs only let the car sit lower unless broken, in which case it sits LOTS lower and rides real strange, dry, wet, icy, whatever.
If you have good tires and the pressures are correct, you could have several other problems, or combinations:
Are the wheels original, or if replacements, are they exactly the same offset as orginal wheels? Those "wide ride" offset skinning things popular around here a couple years ago will ruin the handling, doing exactly what yours does, since the suspension is set up for zero offset and you now have LOTS of it.
Second, do you have any torque steer (pulls one way on acceleration and the other on deceleration) or any thumps or bumps from the rear? Usually accompanied by the feeling that the rear of the car is moving around on the subframe. This is bad links in the rear suspension, allowing the toe on the rear tires to change under load. Drives terrible, and wet roads get scary. Seriously overinflated rear tires make the back end squirrely, but won't thump or torque steer.
Last, worn tie rod ends on the front or a poor front end alignment will make the car VERY sensitive to crosswinds and road surface. A proper front end alignment requires the use of a spreader on the front wheels to pre-load them out. If one is not used, the toe-out will too high, I think, and the car will dart all over the road.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!