91 TSI AWD Problems

I just got a 91 tsi about a month ago and there are a few problems i could use some help with.

First one is the screw on my throttle body has gone missing. I think i heard it fly off, but i was wondering what that screw is called. I believe it is for the idle. I need to get another one, so knowing what it is called could help. The car also idles from 1300 to 1700 then drops down over and over. I was wondering if this could be the cause, it was doing this before the screw came off also, but im guessing since it was lose enough to fall off, then it wasnt adjusted correctly to start. Right now i have an eraser in the opening with a little hole in it to keep it from stalling. also i filled the car with coolant so its not that.

My second question is about a small oil leak. There is a build up of oil just after the oil filter, it goes from the bottom of the part where the filter connects to a little above the filter. It sint the filter though, i check and it is on there nice and snug. It just barely leaks, so i was wondering if you guys had any ideas where its coming from.

Lastly I sometimes, rarely really, but it happens, i have a problem where the car will not let you go past 4 grand RPMs while boosted. So ill come from a stop, have the car reach 4 grand while theres 7psi boost and it sputters and coughs and shakes, all the good stuff, and wont go anymore. It rarely does this, but its happened.

Thanks, any help is appreciated

That scew is the Base Idle Set Srew (BISS) it is necessary and should be replaced as soon as possible. You may need to look into it further because it shouldn't just come out like that. As for the other problem, have you done boost leak test?