Hmmm, that's a decent load so you can eliminate that issue. I've had the #3 since March and have never had an issue with being able to peak at, or slightly above, the 250 setting.
A few things to check before you call customer service:
1. If you're using an extension cord, try it without it.
2. Visually inspect the element and smoker box to see that they seat properly.
3. Consider using a water pan to help stabilize temps on your next smoke.
4. Run an empty chamber smoke and set it to 250; use wood as you normally would and measure temp every fifteen minutes through a 3-4 hour session.
5. Calibrate your Maverick using boiling water, just to be sure.
If you're concerned I know Steve will get back to you in short order to help allay your concerns and come up with a solution if a hardware issue exists.