hello ,
PIX 515E supports a maximum of 1,30,000 connections. since there are close to 49,000 NAT translation entries, and if each xlate has 3 connections, it almost its the max connections that the PIX supports. Under this circumstance i think it will start dropping packets...
anyway you can decrease the xlate timeout value, which can clear out unused xlate connections... other than that, i think the hardware designed for such a high userbase is not correct. for 2200 users , you should have probably looked for a higher end PIX or ASA. also see if there are any virus problems on ur network, which has increased the xlate table...
hope this helps.. all the best.. rate replies if found useful..
Raj