I ride both shimano and campy 10 speeds. I have built up both bikes. The Shimano is far from finnicky, it is very straightforward. Totally readjust the derailleuer, the problem could be coming from many places.

Bent Hangar?
Bent Derailleur?

If you can look at the derailleur from the rear and it looks totally straight,

Is the derailleur very clean and lubed at all points? Does it move freely without the wheel? Shift up and down and watch the action to see if it slides easily. Does it miss gears both shifting up and down? If not, It could be some restriction of the cable if it only misshifts on the upside.

Last but not least start over with the adjustments. I had a similar problem w/a new campy 10 speed installation. The problem was the poaitioning of the limit screws. Make sure you can easily shift to the bottom gear, shift upwards through 10 detents (count them) and make sure that the chain has travelled easily to the highest gear and is all the way onto the outside cog. You may want to loosen the inside and outside limit screws a little extra, you can allways tighten them up later. Run through the gears a few times and count the detents(clicks), make sure you have all 10 and that the derailleur switches gears each time. The move to the 4th gear from the top, adjust the derailleur to fit that gear perfectly square.
Shift up and down, you should have all 10 easily. Re-check the limit screws so you won't dump the chain or get into spokes.

Run the gears up and down, listen for noises, sight the derailleur to the chain cogs, if the chain is centered on the gear, excellent. If it is touching one side or the other, adjust. Keep running up and down watching and counting gears. Listen for misalignments. Once you have the fourth cog, if you are in the proper detent on the shifter, it's usually child's play from there.

Let me know how you fare.