blue screen

my windows 7 64 bitlikes to blue screen when i put it to sleep, happens about half the time, hers the info it give me.

Problem signature:
Problem Event Name: BlueScreen
OS Version: 6.1.7601.2.1.0.768.3
Locale ID: 1033

Additional information about the problem:
BCCode: a
BCP1: 0000000000000000
BCP2: 0000000000000002
BCP3: 0000000000000001
BCP4: FFFFF8000308892C
OS Version: 6_1_7601
Service Pack: 1_0
Product: 768_1

Files that help describe the problem:
C:\Windows\Minidump\090113-20482-01.dmp
C:\Windows\Temp\WER-37284-0.sysdata.xml