My guess is a grounding problem on the amp. When you touch the strings you are shielding the guitar and pickups from noise with your body, which is grounded. The symptoms you describe are very common as, even with a perfect amp, there will be less noise when touching the strings. But what you describe could be a problem in the guitar (unlikely), amp (more likely), or with the plug point (also likely).

guitar problem would be:
there is no shielding. Only the strings and negative from the signal are connected to the negative on the cable. (not likely)

Amp and Plug problem:
ground disconnected/ 2 pin plug etc.

Thanks alot! Ripped out the ols multi and tested the ground... It was the culprid.. Opened plug and fixed it good. What was weard is it would sometimes be there and somtimes not... But sorted!

Just another question, will running a valve amp without ground for a while damage the amp?... Tubes component?

Iamweasel wrote: Just another question, will running a valve amp without ground for a while damage the amp?... Tubes component?

An amp should operate just fine without ground apart from noise issues, it's just safer in case of a short circuit or surge and will cause your power to trip if it is grounded.

It is just safer to have it grounded.