I have a 87 chevy R10 V8 350 truck. 3 weeks ago I started my truck just fine. Just the other day I went to do start the truck and it wouldnt crank. I charged the battery and installed a new alternator. It still wont crank! I put fuel in the fuel injectors and it started. I can let go of the key every time and it will run up for 3 to 5 seconds then die. What can be causing my problems.