Windows 7 Intermittent Headphone Sound Issues After Booting Windows 7 on Asus P6T Motherboard

Can anybody advise please on a sound problem that I've lived with for years and never found a solution to, even though I listen to music a lot. The problem only occurs when listening on headphones, the speakers are ok. I tend to leave the headphones plugged in to avoid disturbance but there's never any sound through them when I first boot up, even though the volume is up and I can see that there's output on the volume control. If I unplug them I get a Windows message confirming that they've been unplugged and also when I plug them back in so the system must be recognising that they're connected. Then if I spend about 5 or 10 minutes repeatedly unplugging them and plugging them in again they will eventually work and the sound will suddenly come through ok. It looks like it's a bad connection on the socket but when they start working they stay working and there's no noise or break up if I move the plug around in the socket, the connection seems very solid. They stay working that is until I turn the PC off and then have to go through the process again when I boot up next time, even though I never touch the connection after it's been working perfectly for some time previously.

System is running Windows 7 32bit with 4GB RAM, all updates have been installed, mobo is an Asus P6T, all drivers appear to be up to date and working and there are no warnings in Device Manager.

Many thanks.