1986 nisson Stanza Mini van.C20 4 cyl.

Motor starts fine, after normal warmup, it will go into "very lo-power mode for about 5 minutes, then will go into normal power mode and will be fine for about 5 minutes, and go into very weak power mode, hard to pull away from stop sign,
just like if the fuel pressure dropped about 10lbs. Or
you disconnected the vacume advance. It has a new fuel pump and filter.I have piped around the vacume switch for the egr and dist, didnt make a differance.
do they have fuel pressure regulator problems with these engines. In weak power mode, you have to depress the accellerator a lot to keep it moving. Could it be the altitude compensator modual.I have the vacume and fuel diagrams. During weak mode it misses slightly. During normal mode, very smooth. Idles very rough in weak mode. Thanks, Bill