How to diagnose engine problem?

My SC430 was running perfectly, then began to misfire. A scan showed a misfire on cyl 5. Checked plug, wire, coil, could find no problem. Couple days later running rougher, scan showed misfire cyls 1, 3, 5, 7. Couple days later, refused to start. Acts like it would if engine immobilizer had been activated: runs about 1 sec, then shuts off. Scan showed problems with both cam position sensor and crankshaft position sensor circuits. Replaced cam position sensor: no change. Reluctant to believe crankshaft position sensor is bad due to progressive nature of faults. Any suggestions would be greatly appreciated.