My 97 K1500 4x4 5.7L 176,000 miles has not started on two occasions (three weeks apart), most recently yesterday. It starts later and is ok, but seems to crank a little longer before starting (all the time). It also backfires when starting (very infrequently - less than once a month).I have replaced the crank poision sensor and fuel pump relay, as well as recent cap, rotor, plugs, wires, fuel filter. My system doesn't hold fuel pressure, with key on (but not running) pressure goes to 45 psi, but drops to below 10 psi in about 30 seconds. Any ideas?