for the life of me, I can't figure this out... 1998 Tahoe, 2-door, 125k miles "Truck runs well cold. Starts great nice smooth idle. Truck warms up still runs great for the first 10 minutes plenty of power etc. After a total of 25-35 minutes of run time the truck surges terribly at idle, and has no throttle capabilities. -Not drivable. Truck surges: RPM Should hold at 625 It almost dies then ramps up to 800 or so. Base timing should hold 0 degrees. It holds 0-1 when cold fluctuates up to 7 when surging. O2 sensors should fluctuate .2-.8. Primaries fluctuate somewhat when surging the post cats stay low often at .000 Fuel pump should run at a constant speed. Hear it grind down as if the system is up to pressure-similar to when turn the key on before starting. Fuel pressure should remain at 55-60psi. It holds 55 psi when cold. It shoots to 60 then down to 40's when surging at idle. It jumps to 60 then flutters in 40's when trying to hold throttle. Exhaust does not seem to plug up, seems to flow OK at tailpipe. ( I have no official way to gauge at my house) Motor/Radiator has about 1000 miles Distributor/Cap/rotor/wires/plugs/#8 Injector have less than 200 miles. Issues started here before installation of below. Fuel pump/pressure regulator/fuel filter/fuel injectors have been cleaned all have less than 50. Crank sensor and relearn have less than 10 miles. Chevy dealer has been looking at it for 3 days without being able to diagnose the problem. Thoughts: Fuel pressure regulator is run on vacuum in intake. The map sensor holds a steady 10" hg when cold. Fluctuates 7-10 when surging."