Kinda sounds like it's starving for fuel.Have you checked the fuel filter.Fuel pump may also be going bad.
Have you checked/changed the air filter.
Does it have a catalytic converter,it may be stopped up.
Is the timing good,has the timing chain ever been replaced.
There's lots of things that can go wrong that can cause this.How did it start.Did it just slowly come on or did you go out one day and it wouldn't run?
