Deere 120C misfire at idle and low load

(Note, I called the problem a misfire because that is what I think it is, but my assumption may not be correct).

Hello,

I am experiencing a problem periodically with my Deere 120C (2003 model). The issue is that sometimes (now, with increasing frequency) when the engine is at idle or under very low loads the motor doesn't run smoothly. I can feel that the engine is running roughly and there is an audible difference in how it is running (I think of it as "chugity chugity chugity" as opposed to a normal "whirrr" sound). When I put the motor under load (digging or moving, etc) the engine runs as normal. As soon as I stop digging for a moment it will revert to the rough feeling/sound.

I changed the air filters, oil, oil filters and fuel filters about 50 hours ago or so. I checked out the engine the first time this happened and everything seemed fine.

I am not sure if it is coincidence or not, but it seems to happen more often when I am facing up hill on a slope. This has lead me to think that maybe there is some contamination in the fuel or something that is getting mixed up and effecting the performance of the engine.

Anyone have any ideas on what the issue may be or have any thoughts on how to diagnose?

Thanks!

Chris

Deere 120C misfire at idle and low load