My 97 Civic is down on power and it doesn't shift gears as it should. Any ideas? on 1997 Honda Civic

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it seems to be worse after it has run a while and has warmed up.
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Is it a V-TEC 1.6? When was last oil change and is the check engine light on or have you noticed the 'D' light flashing at all?
Possible restriction in exhaust. Best would be to have it scanned when the symptoms are present, especially if there is no check engine light.