Need some help, at my wits end.

Significant others vehicle is having a VERY intermittent issue.

1995 Buick Century - 3.1L w/ ATX - 117K miles

Initial startup, regardless of hot or cold can potentially(like I said, intermittent) cause a 'sputter, sputter, chug, chug, backfire out of the intake, stall.

Car will runs flawlessly the entire drive, unless;

Let the vehicle idle for an extended amount of time, 10mins+, same symptoms as above and no amount of throttle will keep her running.

Fire her back up, starts and runs like nothing ever happened and will run fine.

What i've done to help troubleshoot;

Fuel filter
Air filter
PCV
Vacuum gauge(19in/hg at idle)
Cleaned the following;
MAF
Throttle body
IAT
MAF Screen/housing
IACV
Vehicles cooling system works flawlessly and was recently serviced.
'Seafoam' the vac lines, ran an entire can through her to clean her out.

Has spark on EVERY cylinder when the problem is occuring. Every coilpack is in working order.

Have not checked the fuel pressure yet.

Car does not leak or burn any fluids and is mixing no oil/coolant.

It confuses me for the fact that the car runs absolutely perfect all the time, other than when this occurs.

Please, help me troubleshoot this, I need to know where to look.

Wiring? ECM? Fuel pump? EGR?

- Drew