Windows 98, XP? Are you going for the full classic experience including hardware and OS?
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Some things to try:
Have you tried different resolutions, both your system display and the game?
Windowed mode vs Full screen?
Only running with one gfx card (try with one in and then swap to the other)?
Try with different memory sticks or in different configuration
Try replacing the eqclient.ini file with the default in case some display settings are mucking things up.
Some files may be corrupt, maybe try re-installing eq and/or running a check disk
Ensure that your P99 installation image is valid
Good luck hope you get it sorted.