what year is your car? if it's a speed density car, check the vac line. my Tbird's vac hose came off of the MAP sensor and it was running pig rich. it would really only run if it was at WOT, and sputter like yours. i thought all the vac lines were good until i checked them all over again (same thing happened to my mustang, it was running lean and one of the vac hoses was disconnected, i coulda sworn they were all connected...)
'65 Mustang Coupe - currently being rebuilt/restored
'08 Buell XB12R - also being rebuilt (crashed it)
'95 Acura Integra - grocery getter
It depends on what happens to the regulator. If it gets stuck closed, then the fuel pressure will go through the roof and the engine will run very rich and will probably bog quite badly off idle and won't pass emissions. On a N/A car, it will probably run like a pig across the entire powerband. On my SVO it would actually run well at WOT because the fuel pressure was about right for 15 lbs of boost. If its leaking it will probably run a little rich but nowhere near as badly as if the pressure is too high.
Speaking from experience, the best way to diagnose any kind of regulator problem is to hook up a gage and see what the pressure is at idle. When mine was stuck closed, I was getting 60+ psi at the rail!!
1986 1D SVO - SOLD 2007 Mazda3 Hatchback 2.3L beater 1999 Jeep Wrangler 4.0L Sport