Server being slow, cant find the problem

Hello,

We are a starting webdesign company and have our own VPS server for some months now. But the server is getting slow everyday, i tried looking for solutions in these forums but couldnt get a straight solution.

Some facts :
The server is usually fast again after a gracefull server reboot and a MYSQL restart ( mysql wont work after the reboot? )
There is a lot of spam mail going to mailboxes since the start of this server ( didnt have that problem before )
A customer has had a spamattack (?) since saturday where hes getting emails from Postmaster@126.com and mailer daemons every minute. I tried stopping it somehow but I couldnt find a real solution.

Any help would be appreciated !

This is our server info :

Code:

Processor Information
Total processors: 2
Processor #1
Vendor
GenuineIntel
Name
Intel(R) Xeon(R) CPU W3520 @ 2.67GHz
Speed
2660.059 MHz
Cache
8192 KB
Processor #2
Vendor
GenuineIntel
Name
Intel(R) Xeon(R) CPU W3520 @ 2.67GHz
Speed
2660.059 MHz
Cache
8192 KB

Memory Information

System Information
Linux server.########.be 2.6.32-042stab061.2 #1 SMP Fri Aug 24 09:07:21 MSK 2012 i686 i686 i386 GNU/Linux

Physical Disks


Current Memory Usage
             total       used       free     shared    buffers     cached
Mem:       1310720     632604     678116          0          0     384992
-/+ buffers/cache:     247612    1063108
Swap:            0          0          0
Total:     1310720     632604     678116

Current Disk Usage
Filesystem            Size  Used Avail Use% Mounted on
/dev/simfs            120G   35G   86G  29% /
none                  640M  4.0K  640M   1% /dev

 

Thanks.

I contacted some companies and will update this thread if I have found a solution for our problems.