You may need to replace the switch. If it works intermittently, and the green run light comes on when the button is hit, check the following things:
Remove the needle plate and clean thoroughly in the bobbin area then give it lots of oil.
Oil every other oiling point a lot, not just one or two drops but 8 or 10 drops, then let the machine sit and run unthreaded for about 10 minutes. Just set the dial to somewhere on the stitches per second side of the dial and let it run to get everything thoroughly lubricated and warmed up.
If you aren't currently running the warm up procedure each day before you start quilting, do so from now on. You'll find information about that in the manual.
If after doing all this you still get the hesitating, go into adjustment mode and run the needle speed auto-adjustment. You can check the regulated stitching speed too.
You can find instructions on running the adjustment in your manual or at www.intellistitch.com.
Check the sensor wheel 'o' rings. If they are deeply cracked or broken, they will need replacing. You can buy them from http://www.quilttech.com/ .
Check that the sensor wheels are clean and turning consistently as you move the machine. Look for thread caught around the sensor wheel shafts. Lift each wheel in turn and spin it to make sure it's turning freely.
In addition, check if you have the machine set to constant speed with motion detection. If the machine beeps twice when you start up, you have motion detection on and the machine will not start stitching until you move it. To get out of the motion detector mode of constant speed, hold down the black button on the handle as you power up.
If none of these solves the problem, your switch is probably shorted. Replacement switches can be purchased from Radio Shack or www.quilttech.com. For instructions on replacement, go to: