My 1999 5.7 starts fine,go about 10 to 12 miles loses power turn key off for 15 sec. start again,get another 10 to 12 miles

check for a lose wire in the fuse box or a bad conection

Have the crank position sensor checked. This sounds very familiar.

Many things could be your problem but i would say your coverter is stopping up, or ECM "computer" need's reset., You can do this yourself by turning the key to on position turn on a/c then disconnect battery cable " POSITIVE" wait 30 sec. turn key off and connect battery back, Good luck hope this helps some but it could really be many different things.

It sounds like you might have a clogged catalatic converter on the exhaust system.

scan car for free at parts place-it will give u a readout of possible symptoms--inspec plug wires and dist/ign area--also fuel pump may be aging--test for loose wires under hood--may want to get an aditional scan somewhere else

The best way to help find the problem, is to have a fuel pressure test done on it, that will tell if the fuel pump is weak, or the fuel pressure regulator is malfunctioning. If the pressure is low, the fuel pump is weak,
if it is high, the fuel regulator is at fault.

fuel pump may be going. try this. turn key on, listen for fuel pump to run for 5 seconds. Turn key off, wait 15 seconds or until you hear relay click inside car. Turn key on again for 5 seconds and ff for 15 seconds. repeat once more. This will pressurize fuel system to 40 psi so injectors have full pressure. If it starts quickly now, you are getting bleed down through the check valve in the fuel pump or the fuel pressure regulator on the fuel rail.

check all of the vaccum lines and check the timing and see if they have any thing at the auto store for diesel to put in the tank you may have also got a bad tank of diesel fuel. i also had something like this once happen to me and the tank i got it was not diesel someone had put unleaded in that tank by mistake

It sounds like you have some **** in the gas tank that is being drawn to the pickup blocking it.
Have seen this and because it seems so random, it can drive ya nuts.

You might carry a coffee can with you and pull the line from the fuel rail when this happens stick it in the can and just have someone turn the key on for a few seconds.
Beware though, it comes out with force.

Also possible is a failed fuel pressure regulator.
Those aren't expensive and . . . well, not very good either.

My 2004 CTS started doing the same thing last week (October 24) Took it to the dealer the PCM was shot. It was covered under the factory warranty (130 000km if I remember right) works great now.