starts and runs great.. then idles really rough
 

Just picked up a 94 blazer, vin w. it was running rough so i took off the intake and had a leaking fuel line, i replaced the line and cleaned up the intake. it ran great for about 5 minutes and started running really rough, surging, missing, and when i stepped on the gas I had no power at all. If I shut it off and restart it starts right up and runs great. for about 10 seconds to a minute, then runs horrible again. i rechecked in the intake, no leaks, no gas, no vacuum leaks i can find. and it runs great and starts easy every time i try to start it, but only for a very short time. any ideas would be great, im going to get a pressure gauge and check fuel pressure, but im fairly sure thats not the problem.... if you have any ideas I would be happy to hear them. thanks!