Display on TI-83 Plus fills with "rotated" equals signs

Display on TI-83 Plus shows lines (like equals sign rotated 90 degrees) in the top rightmost character postion. When you press enter multiple times (moves cursor to bottom of screen), the screen fills up with these vertically oriented equal signs in all the character positions - like a bunch of 11's in each character position. There are distinct gaps between columns and rows (i.e. not continuous lines from top to bottom). The rightmost column's characters appears to have something else displayed in that column in addition to the "equals".

Installed fresh batteries, reset calculator - still same result.

Thoughts?