2004 STi - Vibration while accelerating in a narrow rpm range

[STiBlammo]

Hello, I very recently bought a 2k4 STi which is all stock except for a performance aftermarket clutch of some sort with less than 3k on it. The clutch install was done by a Rally shop (Rocket Rally in Squamish, BC I believe) which builds world class Subaru rally cars and will only install parts they provide themselves. While accelerating slowly from about 2500rpm to 4000, I can feel a shudder-like vibration similar to what you would feel when going into gear with a clutch that is slipping, only the vibration is less pronounced and while under acceleration for small 3-400rpm ranges, sometimes a couple of ranges in a single gear, all close to 3000 but slightly different in the different gears from 3rd through 6th. I cannot feel the vibration as much in 1st or 2nd, 3rd would be the next least noticeable increasing to 5th gear and then in 6th gear it decreases a bit.

Motor mounts are the best diagnosis I can think of, but I have next to zero experience with Subarus, so any constructive insight would be very helpful and very appreciated.

I have also felt another strange glitching type feel while rapidly accelerating in 2nd gear that will happen repeatedly until shifting or letting off the throttle. The "glitch" moments are separated by about half a second at WOT or close to it and are accompanied by a very brief lag feeling at the instant of the glitch with a click/knock type noise.

I have only expierienced this twice and don't have what I feel is a complete description yet, so I will add if/when I experience this again.

A trip to the local dealership is also in the plan, but I literally just got the car yesterday and drove it back from the city I bought it, so I haven't had a chance yet. But more pertinent information is always good.

Thanks in advance for any help anyone can provide.

[STiBlammo]

The second expierence describing the effect while rapidly accelerating through 2nd is much more noticeable as the starting rpm is decreased therefore going through a larger range of rpm during the 2nd gear pull. The noise/feeling experienced is a knock or clunk-like noise which sounds to be coming from the mid-rear region of the car.

Has anyone experienced any similar symptoms?