Hello!  i have had my GS for 1 full year now, 6,000+ miles.  I have done 4 oil changes in this time along with some minor up keep. 

I have noticed a problem with my GS, its kinda wierd because it only happens when i have my girlfriend on the back with me.  Last night i was riding home with my gf, we where going about 90mph on a flat, straight, open road.  After about 4 minutes the bike started to sputter like it was running out of gas, BUT i had just filled it earlier that day.  I tried putting it on res. and prime, but it made no difference.  After sputtering/bucking for about a minute it just randomly died!  I immediatly tried to start it back up, but no go... 

I pulled to the side of the road, put the bike on the center stand and checked the oil and gas, both looked fine.  So once again i try to start it, all it did was try to turn over but never actually started...  After about 5 minutes it started back up like normal and rode home like any other day...

I find this weird because the only time the bike sputters or dies, is when i'm riding 2 up and going fast...  I was thinking it might be a petcock issue, but that wouldn't explain it not wanting to start for about 5min. would it?

BTW- i only weigh 135 and my girlfriend is far under that....

Any ideas??

You do use more fuel with extra load. You may run the same speed and rom but you are doing it at a bigger throttle position.
You may have a vacuum lock that only can let in air into the tank at a certain rate, and exceeding that rate = vacuum lock = die.
Cool.
Buddha.

So would running the bike on the Prime position solve this issue?

Not unless you let enough time go though and vacuum resolves itself. After which more by luck than anything else it may never occour.
Cool.
Buddha.

Well I dont have much knowledge on the carbs but check the intake and make sure you didnt suck something in it which would decrease airflow or it could be because the bike runs lean already.  I know Buddha can put some weight on what im saying.  What do you think Buddha?

The next time it dies and won't start open the fuel cap and leave it open and try starting the bike.  If you are having vacuum lock then it will start with the cap open.  In that case you can try and clean the vent hole on the cap with a pipe cleaner or something and that might help.  If not you may need to replace your fuel cap.

Riding on prime helps, but if like Buddha says your tank has trouble breathing, it will hold back the gas from the carbs.

Try this.  Ride it on prime and if the problem happens again, open the gas cap then close it.  If it starts right up in mere seconds, your tank vent is plugged.  If your tank vent isn't plugged, it should run fine on prime.  Some Seafoam or Marvel Mystery Oil in your gas will lubricate your vacuum-operated petcock valve, helping it to work better so you can ride with the petcock on any position.  Worked for me.