Volkswagen Repair: RPMs going up and down, gas additives, vacuum lines

Question
I have a 92 Passat and the car always kind of had a wierd smell to the exhaust since I bought. I didn't think much of it, I ended up putting a custom exhaust from the cat-back, and everything was fine for about four/five months. Then all of a sudden she started to rev up and down at idle and when you push the clutch in. Also she still had a wierd smell to the exhaust. I haven't ran the car in like 4 months, but I would like to know what the problem could be and if its worth fixing.
thanks

Answer
If the nameplate on it said Chev or Ford, would it be worth fixin?  It's a car, thats all.  Your other choice is to walk.   
  So, lets look at it as a car.  The reving up and down  ..the car is sucking air and that air is getting under the throttle plate.(just like you were stepping on the gas(which in fact  lets in more air) so first place I would look is vacuum lines and how tight they fit to their connectors.. pull off a vacuum hose and look at the end of the hose--is it a nicely cut off end or is it somewhat flared and broken around the edges?  Pinch the end between your fingers and see if it will squeeze shut(like a rubber hose would)  If it is too hard--replace the hose for it is not fitting on tightly enough to the brass end that you pulled it from.  Also look for abrasions along its length or breaks --the hose should be flexible not brittle.  Also check all clamps and connectors that have to do with the intake side and the nuts at the base of the throttle plate.
  If you still can't see the problem--get a spray can of disc brake cleaner and while the engine is running spray around the parts I just mentioned especially at the ends of the hoses and in kinks or bends and the gaskets.  If the engine stumbles or accelerates then your problem is there.
   Weird smell to the exhaust?   Did you recently put in a new exhaust system?    The gas companies put additives into gas during the cold months so that the gas additives still work.  Could be that or a bad batch of gas.  Wouldn't worry  'bout it though.