aj
i had an older style waist harness which did the same, ride up float all over the joint. Could never get it tight enough for it to sit still. I thought now's the time to stop being a tight arse and invest in a new one. Best thing i did. easy to adjust and firm.
Drop into any windsurf store in Perth you have at least 4 to choose from and try them out. well worth the investment.
If you follow subsonics tip you might need to invest in a bigger board. they cost more than a harness i think