Update your post to include full system hardware specs and OS.
Look in Reliability History for error codes, warnings, and information events that correspond with the crashes.
Use Task Manager or Resource Monitor to observe system performance. You may spot something changing that leads up to a crash.
Run the built in Windows troubleshooters. The troubleshooters may find and fix something.
Power down, unplug, open the case. Check that all cards, cables, RAM, and jumpers are fully and firmly in place.