Every time a freeze while beeing ingame

Hello, i 've got the following problem, everytime i'm ingame after 20-60 minutes i get a freeze ingame. When you are playing League of legends 5v5 it's very problematic when you leave there xD
This is my system:
CPU: Intel Core i5 2500k
MB: Asus P8Z68-V/Gen3
CPU Cooler: Scythe Mugen 3
RAM: TeamGroup Elite DDR3-1333 DIMM CL9 16GB (4x4)
Power: 530W be quiet! Pure Power CM BQT L8-CM-530W
Video: 1280MB Gigabyte GeForce GTX 570 OC

i also tried Memtest and my RAM isn't defect.
i used systemfile checker and there isn't anything corrupted

This is my eventlog
- <Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
- <System>
<Provider Name="Microsoft-Windows-Kernel-Power" Guid="{331C3B3A-2005-44C2-AC5E-77220C37D6B4}" />
<EventID>41</EventID>
<Version>2</Version>
<Level>1</Level>
<Task>63</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000002</Keywords>
<TimeCreated SystemTime="2011-12-15T19:04:05.194010000Z" />
<EventRecordID>16219</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="8" />
<Channel>System</Channel>
<Computer>Mali-PC</Computer>
<Security UserID="S-1-5-18" />
</System>
- <EventData>
<Data Name="BugcheckCode">0</Data>
<Data Name="BugcheckParameter1">0x0</Data>
<Data Name="BugcheckParameter2">0x0</Data>
<Data Name="BugcheckParameter3">0x0</Data>
<Data Name="BugcheckParameter4">0x0</Data>
<Data Name="SleepInProgress">false</Data>
<Data Name="PowerButtonTimestamp">0</Data>
</EventData>

if i type this error code in google i get 99999999²+1 threads about this, but i don't know realy what to do

Check your CPU and graphic temps. Are you having problems with it locking up any other time or just with this game?