A bit of oil in the intercooler is normal but if there is oil there could be air leaking out too, a shitty top mount itself won't change boost from day to day, try tightening all the clamps, there are 4 + the BOV, check all the vacuum lines for cracks too but it's probably electric, have you checked for engine codes, they are stored in your computer and easy to check, just search on here for check engine codes.

With the stock turbo it shouldn't start to lag at 3500.. I think you have a faulty AFM with an intermittent failure to read the airflow. When it lags does it seem to run fairly rough?

Step one is to inspect the plug and wiring and get a can of AFM cleaner and the screwdriver you need to get it out.

Step two is ring a workshop to confirm its your AFM before you drop a couple of hundred on a new one