2001 sentra se idle question

After my car has warmed up, and idles around 750 rpm, and I come to a full
stop, the idle is smooth for 10 seconds. Then the engine stumbles, the idle
dips down to 250 rpm for 2 seconds, almost stalls, then the idle goes back up
to 750 rpm. I have new spark plugs, spark plug cables, clean air filter, (
there is no fuel filter, except for the one inside the tank(brillant, NOT!)).
5 speed manual, and my mechanic (at a Nissan dealership) claims to have tested
the fuel pressure, and sez it's ok. He wanted to change some valve for $369,
but said he wasn't sure if that would fix the problem, so I declined. Any
ideas?
-Don