My 1999 Nissan frontier 4 cyl. stalls out at times. It will restart then shuts down again. At times it runs fine. I have replaced fuel pump, fuel filter, distributer, spark plugs & wires.

no VIN posted or country and means ,impossible to know this.:
he Nissan Frontier is a nameplate used on two different pickup truck models by Nissan:

• Nissan Frontier (international), an alternative nameplate for the NP300/Navara on some markets ,SOLD UNDER 15 NAMES WORLD WIDE;
• Nissan Frontier (North America), a rebadged NP300/Navara from 1997 to 2021, then became a separate model since 2021
• Sold as D22??????canada or USA?

VG30 V6???
VG33 V6???
the Nissan here has a cam timing belt, if allowed to slip at 60 to 100kmiles the engine is wrecked, valves in the cyl. head bend!

so do you ignore this belt>? if yes it will get worse. $$$$$$$$$$$$
do not disconnect the map, and leave it like this, ever.
forget evap code until engine is fully serviced.
all Nissan's are interference engine,<< belt never IGNORE
1: check the cam belts for fray,cracked or skipped time.
2: new spark plugs (before o2, fuel cannot burn with bad spark.)
3: check fuel pressure to spec, FPR can be bad. and low pressure.
4: pulling map can kinda hid a bad FPR, if pressure is low.
5: new distributor timed wrong. as most noobs do,
6: clogged 1/2way injectors.

The restarts?
That sounds like the classic symptoms of a failing fuel pump

you do not say wether check engine lite is on or if there are codes, the fuel pump may be the culprit and be worn out and getting lazy, which can cause this problem.
Fuel pump pressure can be checked with a gauge to verify condition of pump.

Have you checked the mass air flow sensor for contamination? Sounds as if the fuel pressure is adequate,but you have another sensor going out, Nissan's air flow sensors out more common problem.

Sounds like the module inside the distributor. Have to be replaced.

Timing belt

first of all replace the fuel filter and ck if the fuel pump over heats if its hot replace pump too.

I have a 2000 nissan frontier crew cab. Great vehicle! about a year ago at around 100,000 miles she started stalling and running really rough. I replaced the distributor cap, all the spark plugs and wires...about $85.00 worth of stuff.
She now runs like a champ again. I would recommend this if you have not done it...I have owned the truck since it had about 55,000 miles on it. I had never done any of these things...the spark plugs literally fell apart when I pulled them. The last plug is a total bear...it is all the way at the back very close to the firewall. "You will SEE a small "notch" in the fire wall...this is where you follow the notch down to the plug with your wrench..you will also need a 45degree elbow on the wrench with a long extender or you'll never get it off and the new one back on...it was a pain!
Hope this helps. Aaron

lisen for the fuel pump to run when it acts up. if u cant hear it then replace the fuel pump relay