Display Problem

I have a PC which, when booting up, will display flashing vertical lines on the monitor. It appears to be booting up normally, but the lines obscure the view of the screen, so it’s impossible to tell for sure. The thing is, it’s intermittent. If I boot up with the Win98 Startup Disk, then shut down, then start it again, the problem seems to go away. However, almost without fail, if I shut it down for the night and restart it the following morning, the problem will recur. Any thoughts on this would be greatly appreciated, thanks!