The advise above is right on target. There is no high speed instability characteristic that is part of the W124 chassis by design. If you are experiencing a lack of control at 85 mph, you are driving with some badly worn parts.
My limited experience would say if you do not have a lot of play or pulling in the steering, and the shocks are not doing a good job of damping out movement, the problem likely lies in the shocks. They typically leak a little oil when they fail as well, so peer into the wheel well and see what it looks like.
I drive Michelin H and V rated Pilot XGT4 series tires exclusively after experimenting with Dunlop, Pirelli, Bridgestone and Phoenix (3011's I believe, anyone remember them?) in V and Z ratings over the last three decades. This is the finest tire, consistently, that I have ever driven. Properly inflated, and of the correct size, unless you have an unbalanced tire I do not see how Michelin H rated tires can contribute to the poor high speed stability. Unbalance and underinflation coupled with poor alignment can make the car misbehave very readily. Add the incorrect size, and the problems will get worse.
These cars have 4 wheel alignment procedures to follow, so alignment is not something to take for granted. Losing traction easily is a sign that either the tire is underinflated, or the car is not aligned correctly. Alignment problems can be due to the aging of the rubber parts or physical damage from potholes or curbs. In any case a misaligned vehicle starts out being forced to slide the tires against the direction of travel, so when you turn the wheel and give it some gas, the already skidding tires just skid some more. And if the alignment problem is due to worn out bushings and things, it is likely the added load just makes the alignment worse, and the skidding that much more apparent.
Suspension and brakes are safety features, so, like others noted above, don't put off the repairs you need as it will increase the risk to your self and to others when you drive. Good luck, and, when the car is fixed, enjoy it. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)