Is it the pump or relay

I believe it is my fuel pump but not sure. The car starts up fine, drives great with plenty of power for about 15 minutes and then acts as if it runs out of gas (sputter, sputter, blah) If I wait a few minutes the car starts and runs great for a few more minutes then..... Could it be the filter on the pump not letting the gas get through well enough? Or would a relay over heat? Another though is could there be a return line problem not venting and creating a vacuum inside the tank?
The car is a 1996 highline coupe, there are actually two lines coming out of the pump into the filter and then one going up to the injectors and of course the return line goes into the top of the tank.

Check for power at the pump when it doesn't start. If the pump is getting power than you know the relay is good. If there is no pressure and power than the pump is probably the culprit.

That is the funny thing, it only does it on the road! We have turned it on and let it sit there and idle and it runs on and on.
It only takes a few minutes till it runs fine again. It will restart right away but immediately dies.

i havent had this problem yet, but from how you describe it, it may be a vacuum leak. at higher speeds the vacuum should increase. I'd say try having it parked and keep the rpm higher and check for leaks.

sounds like sugar in the tank to me you may want to swap relays first but if you drop the tank check inside before you put a new pump in

i havent had this problem yet, but from how you describe it, it may be a vacuum leak. at higher speeds the vacuum should increase. I'd say try having it parked and keep the rpm higher and check for leaks.

You have the most vacuum at closed throttle and the least amount of vacuum at wide open throttle. Higher speeds would equal higher RPM's which would equal less vacuum.

I get a pressure gage with a hose long enough to place where it can be seen while driving.

I couldn't tell you how many times I have had a pressure gauge duct taped to the window on a test drive. You get a lot of silly looks but you got to do what you got to do :)

Ok, I thank you all for the replies. The fuel pump was 22 bucks at the u-pull-it and so I just changed it. It still drives for about 10 minutes and then just dies. (it has more power now so it wasn't a total waste) So I switched up the relays (they are all the same!) and unplugged the Oxygen sensor and that did not work.. 10 minutes at about 2000 or so rpms and dead. It does restart after a few and the longer I wait the longer it will run until dies. So I am going for catylictic (sp) converter next. It is the last thing on the list as far as I know. If you all have any other suggestions it would be welcome. Oh, no vacuum leak either.

It's not the catalytic converter! If the converter where plugged you would have no power but it would run and most likely if it where plugged that bad you would hear an exhaust least before the cat when you tried to accelerate.

Maybe you should look into whether or not you have spark when it dies? Maybe you have a cam or crank sensor flakin out.

Cant check spark when it dies. It starts back up! It actually will fire immediately but will again die. If you wait 5 minutes it will run for 2. Wait 20 and it runs for 5 wait an hour or so and it will run for 10 again. It is like the auto shut down is dectecting a problem but I cant fathom what it could be. Could the cam or crank sensors do this? It runs so well with lots of giddy-up right up to the second it dies.
Also, we cant get it to send any codes.