Sadly, it�s fairly common. I guess your �lumpy� equates to what�s known as surging, which prevails in on/off throttle situations and part, or small throttle openings � makes the bike feel a bit jumpy, jerky or lumpy! There is quite a lot you can do to reduce the symptoms and smooth out your XT and to all intents and purposes cure it, but it unfortunately can never be 100% eliminated.
Check the obvious (and cheap) stuff first: Fresh fuel, known good spark plug, clean air filter, correct idele speed (1400-1500rpm), correct chain tension (and if the chain and sprockets are in good nick), plus see if there is excessive rotational play at the rear sprocket (which would signify knackered cush drive rubbers.
There are plenty of mods you can do to reduce the surging, a lot of which are either free to do or cheap and easy. Here's some reading homework for you...
1. Join XT660.com supporters to access the �Ask Kev� 1:1 technical help section -
http://www.xt660.com/payments.php
2. Find out a bit more about your ECU and what can be done to help with surging and jerky throttle response -
http://www.xt660.com/showthread.php?t=8546
3. Twist-grip mod -
http://www.xt660.com/showthread.php?p=154983
4. Cush drive rubber mod -
http://www.xt660.com/showthread.php?t=12422
5. Adjust your CO -
http://www.xt660.com/showthread.php?t=55
6. Adjust your TPS -
http://www.xt660.com/showthread.php?t=56
7. Kev fuelling mod -
http://www.xt660.com/showthread.php?t=194 (or fit a Power Commander which is a LOT more expensive)
8. O2 controller -
http://www.xt660.com/showthread.php?t=14746 (current version from post #165)
Click on the link in my signature for the full list of all the mods available.