monogfx

Hi there - 2003 WRX, pretty standard under the bonnet. Have had the car from new almost! Got 100k on the clock. After a trip down the motorway or long trip, park car get coffee, half and hour later rejoin motorway - only 0.5 - 0.6 bar boost and car feels down on power (normally 1.2). This will last for about 30 mins, then the boost and performance will come back. I have always put this down to heat soak, but is there something else more serious I should consider or other things I can check? Bearing in mind the mileage. Uses no more oil than before, not smoky either.
Thanks
Tim