Hi guys another newbie here. Bought the wrx 2 weeks ago. Problems are as follows:

1) Whilst driving in 4th or 5th at high revs at a certain point the engine sort of jerks forward and cuts out momentarily. At first I thought I had blown the engine. It has happened twice now and since the last time I have been driving carefully to avoid this as it seems un-natural and feels like it could be doing some damage.

2) Engine has just started stalling whilst changing gear. There is no warning sign, the first I noticed it was when my steering became heavy. Engine check light illuminates. This has happened in various gears. Engine starts fine if I turn the key again. Now I'd like to point out here that I have just started using LUCAS octane booster instead of Millers oil. Last night I went to fill the tank, emptied the full bottle of Lucas into the tank then reaslised they had no V-power left. (Silly I know). I put a gallon of unleaded in the tank just to mix something with the Lucas , drove 2 mile to another garage and filled with V-power. Could this be causing this phantom stalling?

Car is fairly standard apart from induction kit, boost controller, boost gauge, BOV and larger exhaust. I am unsure if its has been re-mapped or chipped in the past. It is boosting at 13 psi and 80,000 miles on the clock.
Thanks for taking the time to read this guys any advice is welcomed and appreciated.