When I start my computer, DVD applications & internet calls (Skype) work OK. After a while, movies start stuttering (audio & visual), and the same with phoone calls, I'm told by people I'm talking to.

I've no idea why. Could it be a dodgy processor, as it is OK when I first startup.

Any help appreciated.

Welcome to PCHF

Could be that your PC is running low on virtual memory and the problem gets gradually worse as you use the application. Try setting your virtual memory to "system managed size" like this:

Right click on My Computer and select properties > Advanced > Performance/Settings > Advanced tab > Virtual memory/change > make sure that the virtual memory is set to "system managed size" click "set" then ok and re-boot