I'm interested in a 95 sierra...Owner said its missing out and won't rev over 3000 rpm....He thinks the timing is off...He said he has replaced plugs and wires but not the plugs... What is the most likely reason it is doing what its doing?

I'm interested in a 95 sierra...Owner said its missing out and won't rev over 3000 rpm....He thinks the timing is off...He said he has replaced plugs and wires but not the plugs... What is the most likely reason it is doing what its doing?