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Hello, I hope I can be of assistance.
Have you the chassis number of the car? When the fault occurs are any warning lights or messages showing?
sorry for the delay in replying I've been having computer problems.
You need to get any fault codes read from the engine management to see which sensor or actuator is causing the car to go in to limited operating strategy which is why you are getting the lack of power.
The most common concern that causes the fault you describe is normally a turbo boost pressure problem. The turbo is a variable vane design, as the boost pressure increases the engine management uses a solenoid to control the vacuum to the variable vane actuator enabling the boost pressure to be lowered if it rises too far.
If the engine management can't lower the boost pressure fast enough it will cut the vacuum to the variable vane actuator causing a complete lack of boost pressure giving a lack of power.
Once you turn the engine off the system resets until the fault occurs again.
Check that the vacuum pipe to the turbo actuator is OK and that any pipe work to the solenoid is OK.
As I said t the beginning get any fault codes read from the engine management and act on the code or codes.
Once again sorry for the delay in replying.
Let me know if you require further assistance and I'll reply as soon as I'm able to.