This is most likely something related to the MAF (bad MAF due to oiled filter or leak somewhere after), but I had this exact same issue on my old Porsche 924S and it turned out to be a loose ground cable bolt. I had a similar problem on my GN and it was also related to bad grounding. Just check your battery cables and ground connections. A good test is to take a set of jumper cables, connect one end to your negative battery terminal, and the other to the motor or the frame. Tuck it so you can close the hood and go for a drive. If the problem changes to a higher RPM or disappears, uoi found your issue.