Power problem 2.5 1999 boxster

[J60SON]

Power problem

I got my car back 4 weeks ago after undergoing head gasket replacement
Since then I have had a power problem which is more noticeable the harder i try to drive the car. Up to 2.5-3k rpm I see maybe 25% drop in power, from 3k – 5k maybe up to 40% power loss. I will not take it up any further as I am a bit worried?
There is no other visible or physical signs only holding back
Anyone help?

Jason

[FERRARIFRANK]

double check cam timing and anything else that was touched from the head gasket change

check compression and compare numbers

[J60SON]

The mechanic dealing with this is reputable but he is still not sure what it is, there are no faults showing on the readout?