front end problems

I bought a clean 99 Eldorado last fall with 69k on it.It had a knocking in the r/front.The dealer thought it was the strut and put on a new one.Still had the problem.I took it to the local Monroe shop and they said the stab links ? were bad,so the dealer replaced them.It was ok for awhile.It started to pulsate when I stopped,and it also developed a steady knocking in the r/front.It feels/sounds like an exhaust pipe or something along that line knocking against the floor under the pass feet. Back to the brakes. First they turned all 4 rotors ($200) it was ok for about 4 months.I do a lot of highway driving,car now has 88k on it. It started pulsating again and I took it back to
Monroe, now of course the front rotors are warped again.I was 2 days from driving 2k+ miles on vacation,so I had them replace front rotors($300)they say pads are good.
After vacation,(2500 miles) it was pulsating again. This time I went to a Tuffy shop and had them look at it.They said front and rear rotors are warped.Everything else in the front looked good. I had them replace back rotors and pads($200).I went back to Monroe and they were stumped as to why they warped,but did put on new front ones again.Both shops have said the front end is tight.But I can still feel a shimmy at varios speeds,75 is pretty noticable.I drove 650 m today with all new brakes and still have a pulsation when I apply the brakes.I still have the knocking under the pass floor.I am hoping someone can help me with this.I have over $700 in new brakes and rotors,still have the pulsating,knocking, and 2 shops can't seem to figure it out.
Please HELP
Thanks
Clancy

Noises are virtually impossible to diagnose in this environment, loose sub frame bolts, motor mounts, mis -aligned exhaust and so on. The pulsation can be caused by many things also, excess wheel hub runout, not cleaning the hub mounting surface when installing the rotors. The shimmy/vibration at speed could be another issue, road force variations in the tires, a bad tire, an out of balance tire to name few, these are issues that need to be tackled by a top notch tech who also has access to Gm Tsb database.

My next option was the local dealer.I guess I'll try that.
Thanks