In the early 11.xx (service) releases, there were a few problems with memory management (spikes in memory consumption, especially when using server objects). From your statement "We are using this server version for about a year..." I guess you're not using the latest SR. What version of QVS are you running (avoid 11.00 releases) and are there any objections to upgrading to SR12 ASAP?
Last few releases seem pretty stable to me. SR11 had a very large amount of bug fixes. IMHO you should switch to SR12 asap.
Note that the QlikView Server will grab as much memory as possible, and won't release it when users are logging off. QVS has its own memory manager and therefor it's not a bad thing when the service eats a lot of memory.
Just to stress this a bit more: QVS eating a lot of memory is not a bad thing. Check QMC->System->Setyp->QlikViewServers->Select yours->Performance->Working Set->High. If memory consumption goes over and stays over that high water mark, then the system probably is in trouble. Running at 99% is definitely not a good thing, as your machine will eventually (seem to) lock up.