SQL Memory Issue

We have an issue with a SQL production server where the allocation taken up by SQL climbs to 99% or higher during the day. The memory never gets released when all user activity on the server is at a minimum. We are having to stop/start the service or rebooting the server when the mouse quick responding. This is now occurring every 1 1/2 days.
The server is on a single CPU running SQL 6.5 with NT 4.0 (at current SP level), ArcServe & Raid 5. Three months ago we upgraded the drives and reinstalled all software and rebuilt all the databases. Prior to the rebuilt had some memory issues but now we are severely impacted.

Per Microsofts recommendation we installed SQL SP5a. This had no impact.
Does anyone have any thoughts?