Are you sure your intercooler pipes are not leaking boost. It would explain why you only get it in high gears when the car is trying to hold boost for extended periods of time. I remememer reading it is the pipes into the intercooler that blow. Why don't you put some proper clamps on and try it then? Apparently the standard Seat spring clips are rubbish at high boost. Do a search, I could be wrong, but it would be an easy fix if I am right!