Mechanics just keep taking my money...

I piced this car up cheap. 1997 Cadillac STS. Its a clean car with high k's (almost 300,000 km's). It's survived 12 canadian winters, its as clean underneath as it is all around Couple minor rust spots here and there. But it rides like an army hummer.

I've had it inspected 3 times. The last garage i took it to switched mechanics half way through the job and didnt finish doing what i asked. It was in for an oil change and i asked them to look at a few specific things, listed some symptoms. They replaced a wiper arm and topped up washer fluid, i didnt ask for either, and didnt want either. They also billed me for 7 litres of oil, i thought this engine took 8? I told them the car shakes, told them when it shakes, and told them what it sounded like. They didnt pull the roters to check for warp, and they didnt even glance at the rear end at all. They told me lower front control arm bushings were bad. Someone else a few days ago told me they were fine...... billed me for materials and an extra hour of labor.

So here are my symtoms.

When parked, car running, there is tiny bit of play in the wheel. You can hear it clunk when you turn it left and right 1/4 inch. It doesnt do this while driving, or when te car is off.

Light-medium braking from 40k+ periodicly the steering wheel shakes extremely hard. It does not do this under hard braking. I find myself pushing harde on the brake a lot to make it stop. It also sounds like im dragging a plastic recycle bin under the car, banging around. I also hear this when im going over bumps.

The rear end sways left and right when i hit larger bumps and potholes. Its actualy jumped about 1/3 of a lane on me at high speed a couple times.

I have a very good source for donor parts. a 1994 STS and a 1997 STS. Each car is limited in whats left on them.

I dont know what to do really. I could take it to a more reputable (expensive) garage and hope for te best, or i could figure out through tips and what-not how to look for and diagnose my problems. This is my 3rd vehicle in the last year ive had this problem with. Everyone wants me to buy parts i dont need. Canadian Tire told me there was a rip in my CV boot on my grand prix.... said id have to replace everything. I waited on it, and a while later we had it in the air doing brakes, and i couldnt find this rip ANYWHERE. So ya.... i dont want to spend any more on hallucinogenic auto mchanics.

What to do.....