Lisa I Agree with Moncha, your problem sounds like it is nothing to serious, maybe just a new set of plugs are needed or a coil pack is starting to fail (which is easy to fix in the US because spares are easier to get than here in the UK).
If it was a valve, you wouldn't have days where it ran well

I have had to order an exhaust valve from a Vaxhall Frontera because they don't list anything for a Vehicross because it is classed as a Gray Import.