You said that you thought it was heat, but it isn't? What were your steps that you took to discover this? It might help us help you. Regardless, if you have cleaned out the vents with canned air recently, I would do that anyway. It's an important preventative maintenance step that many people aren't aware of.
Also, I would look in the Windows Event Log/Viewer to see if anything is showing up there that appears at the same time as your freezes. Since it is locking up you don't have an error message; this could be difficult to track down.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…