If this is a new purchase then a full tune up is in order.
Plugs, wires, cap, rotor, fuel filter, oil change, clean the maf, base idle reset, clean the iac, pcv valve and metal screen, run your codes, check and clean the grounds. Spend a day going through her and admiring her from all angles.
After you do all that then it will be easier to track down the culprit. It could be a vacuum leak.
2016 VW GTI 2.0 Turbo
1989 Sonic Blue LX Coupe, 349 ci, TW heads, TFS Track Heat Intake, 75mm Accufab TB, 80mm ProM, 30 lb. inj, TKO, 3.73, 31 spline w/ TruTrac, MAC 1-5/8" LT headers w/ H pipe, Flowmaster, electric fan Moates QH
391 rwhp & 424 rwtq...Mustang Dyno 11.5@122mph