I am assuming you have the condenser version.

You don't mention the location of the lighter/darker sections: is the problem vignetting, is one side lighter or darker, is it an offset patch that is darker (prints lighter)?

It it is adjustable, try moving the lightbulb's position. Sometimes the light bulb itself is to blame and changing the bulb can resolve the problem. Try unscrewing the light bulb a tad just to see if the uneven pattern rotates with the bulb - if it does then the bulb is the culprit. Check that the condensers are clean. Don't use the lens wide-open, most need to be closed down 2 stops to cure vignetting.

A good enlarging meter is a great aid to troubleshooting illumination problems.