Several problems with '91 735IA

Disclaimer: I'm sorry to say but I'm no car mechanic. I'm living in germany (Bonn) for 2 years and decided to buy a *fun* car because compared to Norway, the car prices here are often 1/5 of the price in Norway. Of course, I'm moving back to Norway in 3 months and I'm now starting to experience problems. (It is a 1991 735IA, 130tkm so it's been treated nice.) I was recommended to post in this forum because you are helpful and understand english so please bear with me.

Problems:

1) The steering wheel shakes "violently" around 90km/t, especially when I'm not accelerating but just rolling.

2) The following happened in the last two days: I start the car and sometimes it dies after a few seconds. This never happened before and I think it is connected with the next: I put the car in Drive but push in the brakes so the car is at rest. The car is now shaking just a bit but it feels like there is two frequencies (two things causing the shaking) because each 20 or so seconds, it shakes violently and then dampens again. Also, when the car is in idle, the RPM is a tad below 500...I would guess 480 RPM. Before all this happened it was usually around 650 RPM perhaps a bit more.

3) If I brake twice, the first will behave normally but the second is as if the brake amplifier (don't know the english term) is gone and it feels like when I'm braking with the engine turned off. If I wait 2 seconds, it is normal again.

Possible solutions:
1) I've seen in the forums that this might be because of break pads or badly adjusted wheels. I bought new wheels some months ago but I can't remember if this started before or after the change (yeah, stupid.)
Is there anything I can do to fix this or do I have to go to a mechanic?

2) I think (and hope) that perhaps one of the spark plugs (?) are not working properly. This could (in my car-illiterate brain) cause the shaking at low RPM, why it dies (harder to maintain torque) and the low RPMs itself. I'm thinking of unplugging each plug and listen/feel if something is different. (The feeling/sound should get worse on all plugs expect when I unplug the faulty one, then it should behave normally).
Any other ideas?

3) My first thought was that I was running low on brake fluid (?) but I checked the container and the level is just below MAX. Is there anything I can check without going to a mechanic?

I really appreciate your help. If you need help with building a computer (which is my hobby) or calculating the mass of a black hole (my profession) send me an email and I'll help you out

With humble regards
Odegard