Reporting Services 2008 R2 ReportingServicesService.exe high CPU

I've had a couple of instances where ReportingServicesService.exe is setting at > 50% of CPU.

I've looked run the following query but nothing stands out.

Use  ReportServer
Go

select * from ExecutionLog2 order by TimeStart DESC

Any suggestions for identifying the cause or putting something in place to monitor CPU usage?

Thanks

Kevin