????

I have a 1999 Nissan almera smashing car runs well
owned it 10 years never failed MOT.but only on a
long run after sometimes 30 miles or 40.once 60 it
starts to jump and jerk almost like it,s being starved
of fuel,only happens when I slow down,if I leave a motorway it jumps and jerks till it picks up again.
I changed the fuel filter which seemed to help but the next trip it started again,,,,if I stop in the services for a while it runs fine for a while then starts again.the AA suggested a vacuum in the tank as it hisses when releasing the fuel cap,tried loosening the cap,didn,t help.most annoying,could
it be an injector playing up when hot?Please can any
one help.