I just recently join new company, the use JDE as main business application. however i facing problem on JDE system with use complaint Enterprise server is slow, not sure is LAN problem or Server problem.

Problem:
oneworld Xe user complaint sometime slow server respone exp. went query the database for name it take 8-10 sec (ping to server respone time is 10ms) to display, normally is 1-2 Sec (ping to server respone time <1ms). but usually is happend in end of every month(closing Account)and duration not long around 10-15 minuts each time happending. 1 day can happend more than 10 times.  

JDE Server Spec
IBM WINTEL server Xeon 2.8 X 2 Windows 2003 server
4GB memory (usage is 70%)test using Solarwinds
72GB HD X5

Enterprise Server
oneworld Xe client for LAN user , around 40 Users
Citrix Server
Citrix Client for LAN user around 10 and for 2 branch (WAN) user aound 20 user.
MS-SQL Databases

LAN  Structure
6 L2 Standard Switches, all same, Dlink 1024R+ , 5 Switch uplink to 1 main switch, all is 100MB uplink only from each switch.
All oneworld Xe is use Switch 2 & 3.

My campany have 6 server, other server is Mail, file , Domain server, Deployment server, Proxy server.

I thinking is my LAN bandwith is not enought to support traffic, specially my 1st switch cannot handle all my server traffic, i plan upgrade 1st switch to 24Port GB switch and 2 & 3 switch with GB uplink, do you think this will help solve my problem? or is my server can not handle transection? any suggestion.