Well, my first suggestion, other than the possible heat problem would be to do some research on that ALI chipset. They haven't faired too well in recent times with the Athlon DDR boards. Check to see if there are any BIOS upgrades, or other driver updates for the ALI chipset.
Secondly, I would look hard at your 8500. If this is one of the older ones, they had a lot of bugs in the drivers. Recent driver updates are supposed to make the 8500's really fly.
55-58C under load is a tad warm. I would seriously consider a new HSF. You can get top-notch performing HSF units for about $25. I looked long and hard at the new SWIFTECH 462's and the ALPHA PAL, but at prices in the $70 range for sink and fan, it just didn't seem to be worth it for the few degrees difference they made over high performance HSF's costing less than half that.
With my new XP 2100 setup, I opted for the Arkua 7228. It's a licensed copy of the Thermosonic ThermoEngine heatsink (with some modifications to it) and the heatsink with a 60mm Delta fan costs just $24.99. My temps under load are around 40-42C.
I picked it up at the Heatsink Factory (
http://www.heatsinkfactory.com).
Pay the $9 and get some Arctic Silver 3 heatsink compound as well, it is worth the price.