I had the same intermittent vaccuum leak which acted up every time the weather got cold. Codes were p0171 & p0174.
I finally found it by spraying throttle body cleaner around the intake plenum where it bolts to the top of the intake stack. It disappeared shortly after detecting it.
I replaced the upper intake gaskets & have had no more codes for two weeks now. Codes would re-occur about once a week before fixing.
Hope this helps.
My car is a 96 mystique 2.5 24 valve
