Strange steering/brake issue

My 2003 Auto Mach 1 has developed a strange issue in the past year. If i start the car and drive it it has no issues whatsoever around town, after about 20-30 minutes the brake pedal will get soft and has no resistance until the last inch of travel where it brakes like normal. If i get on the freeway after about 30 minutes of doing 75-85 the steering wheel will begin to shake slightly and get MUCH worse when applying the brakes as if I was driving down a grated gravel road, shaking left and right violently. If i get off the freeway and drive the regular streets the shaking issue goes away but i can still feel it slightly once I get near 50 mph. The last time I drove it in this condition I had to take the freeway home and did not have to touch the brake at all on the drive but once I got off the freeway and braked to slow down, the pedal was ROCK hard and was very sensitive, not the spongy pedal like I mentioned before

I have replaced the entire front and rear suspension except the rack and pinion itself, front and rear brake rotors, calipers and pads, power steering pump too because of a death whine but the issues still remains. Most of these needed to be replaced anyway, I'm the 9th owner and the miles are just over 150k

I have suspected that my brake hydroboost is to blame here, can anyone concur? I've never had this issue before, I also have a 2002 GT with hydroboost but it has no issues. Looking for advice, past experience or help before I buy a reman unit for over 300 dollars

Thanks