When you loose power...this is how I would start(listed from easy to advanced)
Like mentioned, Verify clutch operation.
Check or replace all filters.
Pull spark plugs, has it been burning lean or rich?(white plug end too lean, black or wet plug end too rich)
if lean, check fuel pressure, pump might be going bad.
If rich, air filter could be restricted with debris.
Further checks would be to perform compression test to find any cylinder leakage do to ring problems or excessive wear and or valve timing or valve adjustment issue.
If you really know what your doing and haven't found a problem with any of the test above, then I would move to testing the fuel managment componets (the sensors that send information to your ECM or computer) they all have Specs. for eather ohm readings or voltage readings. if a sensor looses it's calibration, it will be giving the ECM false information causing possible fuel map changes when not required, this could cause power loss as well.
You need a service manual to find the required specs for this.
And ofcourse for those that don't have a clue what I just posted,or the time and tools, have it checked a your dealer.