Hi Ap,

with nearly all scopes there can be contact problems due to dirt and oxids after some years, like mentioned here:

http://www.fullnet.com/~tomg/tekclean.htm

Avoid de-oxid chemicals, though, they make the situation even worse, usually. Use only isopropyl alcohol for the contacts.

Another cause of noisy lines on the screen can be common mode noise on protective earth or common mode noise hitting the screen of probe. Give snap ferrites on the mains and probe cable a try, then. Another cure is to use differential probes.

Once we had a problem with extreme jitter in the trigger delay of a Hitachi scope. Very nasty if you don't know that its the scope which brings the jitter...

Kai