I only mentioned the clutch because you said it was jerking when you engaged it. I wondered if maybe the clutch wasn't disengaging properly, and that was holding the revs down (like sitting there with your foot only half on the clutch). That seems like a bit of a long shot to me, though.

If there's no roughness at idle, then I'd really stop looking at valves/plugs or anything cylinder-specific.

If it were a bent valve, it would only be affecting ONE cylinder, and so you'd hear an unevenness in the idle (a pretty obvious one at that). If the idle is smooth, you need to look at something that's affecting all four cylinders equally and simultaneously. And yes, if it were a bent valve, you'd more than likely be hearing something else along with the rough idle. Ticking/pinging of some kind.

From what you've said, the sputtering/jumping sounds like it's just due to low revving. That is, it sputters a bit cause it's idling at 300rpm instead of 800. And it jerks when you try to engage the clutch as though it's not getting enough gas. Would you say that's right? If that sounds accurate to you, then my vote's still on the fuel system. Filter first, then Pump, are the likely culprits. I'd still check the filter out before I go about replacing a clutch.

If you've been trying this for a long time and still not getting anywhere, it might be time to take it to a licensed mechanic. For a few bucks I'm sure he can diagnose the problem more accurately than some guy sitting on the other side of a computer can.