I've got a 1998 5-speed manual 4-cylinder Frontier. It has started throttle surging pretty badly. It runs smoothly at idle, but once I push on the throttle the engine starts surging, even in neutral. It's not subtle - the surging is bad enough to make the vehicle undriveable with how badly it bucks. No check engine light. Disconnecting the throttle position sensor makes the problem go away (although the CEL does come on then, of course, and it makes the idle high and the gas mileage poor). I've replaced both the TPS and the MAF with no change. I'm driving it with the TPS unplugged for now, but what should I troubleshoot next?