Brian,
I have a friend who is a White's dealer. He has told me that there are a three weak spots in the design of this detector that result in common failure modes.
One is the LCD display electronics, the second is the pinpoint trigger switch and the third is the voltage regulator chip. The XLT is a very reliable system, but when it fails, it is usually one of these three things.
If your XLT is still under warranty, let White's fix it. If not under warranty, I would call the White's service number and ask them for an estimate to repair the display. It should be a reasonable amount, plus White's will give it a thorough check and tuning (if necessary).