Check that you don't have any keys stuck down.
If you haven't, look in the console for the CRT controller board. On there you will find an MC1488 and an MC1489 ic.
Change these for new.
Look on the CPU board in the cabinet. Under the graphics board there will also be an MC1488 and an MC1489. (U14 and U15). Replace these also.
Hopefully, that will sort your problem.