cam sensor or crank?

[superfast]

Any help would greatly be appreciated. I have a 2002 chevy 1500 4.3. It was missing bad last year around this time. I took it to auto zone to be put on there scanner(since its free). They told me it was the crank position sensor. I replaced it and it fixed the problem. It started missing last week again bad. It would go dead and if you disconnected the battery for a while it would crank up to get me to the house. Now it won't start at all. Its got fire and fuel, but it sounds like the timing is off. No one around where I live has a scanner for the OBD II. And the autozone it over 25 miles away to put it on theirs. Now I hear they also have a cam sensor. Can it be the crank sensor or the cam sensor? The crank sensor I bought was from auto zone. So maybe it was a cheap brand or something.. And if it is the crank sensor again what can be causing it to go out. I heard putting in the wrong oil...but that don't sound right...thanks......I put valvoline 10w30....