Several things to check here.
1. Is the PC CPU overheating? download suitable software to monitor it.
2. Are the fans clogged up? clean them.
3. Faulty memory? run memchecker.
4. Faulty Grahics fan? check its clean and running.
5. PSU on its way out or cannot cope with demand? Check PSU fan and see if its getting too hot.
Finally, remove and replace all cards, plugs, etc being carefull to replace them in the same place...