Odd tire wear and straightline instability - 300E

I've got Michelin MXV4 Energys (205/60R15)on my 1990 300E. The outside tread rows on the front tires are feathering. Looking from the top, each segment is sloping from front to back. I know the toe can cause axial feathering, but I've never heard of it happening circumferentially. The car drifts and sort of floats at speed without pulling and the overall feel of the handling is vague and imprecise. I had the tires balanced, rotated, and a 4-wheel alignment done. It's better, but not firm and precise like it used to be. The shop looked at the steering and suspension and nothing jumped out as a likely cause. The tires are inflated to the recommended pressure and I also tried increasing the tire pressure gradually to the maximum sidewall pressure without noticible improvement. The car has 92,000 mi. and I've not had any suspension work done since I bought it at 52,000 mi. I can't say if there was any work prior. Anyone have any ideas?