86 Silverado Q's

[Chevelle454]

I am posting this for a friend.

Some days the 305 runs good, some days it runs like crap. It misses once in a while and lacks power(a lot). Plugs, wires, cap and rotor have all been changed. Timing is alright. Could it be a valve sticking, carb adjustment, or anything else? Could it be emissions, and if so, what could it be?

Also, what year did Vortec heads come out(305 and 350)?

Thanks in advance...