Problem has become more frequent and pronounced over past 2-3 yrs. Dealers replaced coil, plugs, plug wires, distributor & rotor (I think) at different points, to temporary good effect, but problem persists. Problem goes away with stronger acceleration on up-inclines, and doesn’t happen at any speed on the flat or on down-inclines. One dealer’s transmission specialist road-tested car and didn’t think problem was transmission. Dealers didn’t check for codes on car’s computer, as far as I know.
Car is 1995 Mercury Sable wagon (3.8 liter V6, 120K miles) and runs fine, as always, at highway speeds. Recently mpg has dropped, however, particularly in non-highway driving—maybe somehow related? What could be causing the symptom and what’s the fix?? I drive about 6,000 miles a year, 80-90 % on highways, and my sofa is worth more than the car.
Start with a compression test to rule out a tired engine.
Toss a bottle of Chevron Techron in the gas tank to clean the fuel injectors.
The loss of mileage could be a sticky thermostat. Change it if over 5 years old.
Clean the EGR valve and the related passages.
I’m wondering why you referred to it as “front axle shudder.” Has anyone ever said anything about the axles/halfshafts or CV joints? A problem with a CV joint can cause a shudder. So too can problematic engine/transmission mounts. Another thought I had was torque converter clutch shudder.
But then there’s also plenty of other stuff like engine misfires.
I think that you need to find a different shop. Unless its for warranty or recall service (both very unlikely at this point on a '95) there’s no good reason to use a dealer’s shop. Find a good locally owned place and have someone drive it to figure out what is going on.