Surging Idle Only While Rolling...Strange
 

This probem is so strange to me. I have a 98 a4 1.8t and I am having a surging/roaming idle that only occurs while the car is in neutral while coasting or rolling. When I come to a complete stop the idle goes to normal around 800 rpms. But the WEIRDEST thing is as soon as the car even rolls a little bit forward or backwards the idle starts roaming. I can tell it wants to surge while I am in gear because I get a strange "shaking" pedal feel. I am throwing codes, but I cannot remember what they are. I simultanously am also only getting 4 lbs of boost, and I assume the problem is related. I looked for a boost leak but nothing is obvious and I sprayed it down looking for leaks or any sort of strange air/wind.
I hope I posted in the right place, I am sorry if not.
Thanks in advance.
P.s. I have googled the heck out of it and tried researching like I normally always have success with. I still cant figure it out.

I am not sure yet but I feel like the gas mileage is not as good since this problem started. I am trying to fix it so I can sell it, I already have my s4 picked out so please help me! Thanks!