VT220 symptom

[9000 VAX]

Hi, list,
  My VT220 went crazy yestoday. When I was doing something else on my
PC, suddenly the VT220 monitor was blank (black) and the LEDs on the
monitor flashed. The frequency was about 4 or 5 times a second. The
keyboard LEDs (LK201) flashed the same speed as the monitor LED.
Taking off the keyboard and the RS232 port did not solve the problem.
Does anybody know what happened? Is there an easy way to fix the
problem? Thanks.

vax, 9000