1999 Audi A4 Bad idle power loss

My Audi has an idle problem. Let me first start off by saying that this problem is spuratic in nature and can't be set off by a type of driving. When i'm driving the car I may have a loss of power (car doesn't turn off but when depressing the gas pedal I get nothing), this lasts for on average 2 seconds then I get full power back. This return of power is usually accompanied by a burning smell (sometimes).
The other problem I have is when stopped in idle, the rpm gague jumps bellow idle spuratically for upwards of 10 seconds and either returns to normal or completely shuts off the vehicle.
I recently drove the car from home to school (660 miles which is 99% highway) and didn't encounter any of these problems at all! It seems to happen around town in the stop and go.
PLEASE any help would be perfect because i'm trying to sell!
-I recently changed the oil and put in an additive to try and ensure propor lubrication longevity.