I have been digging into my scope for some time, but there is one thing
I cant find, because the fault dissapears when the casing is removed,
seems to be heat related.

WHen the scope is powered up, both channels set for gnd, I can se a
stright fine line for each of the channels, but after som 5 minutes or
so the sweep is distorded, it sort of look like this:
_____________________________
I
\

The difference between the start of the sweep and the horizontal line
is about one div, if I try to write the sweep in XY coordinates it
looks something like this
Start 0.5,-1
start+1 0,-0.5
start+2 0,0
start+3 1,0
Then it is a straight line
This at 20ms
At 10 ms
It looks like this
_______________________________
I
I

And then the faster the sweep goes, the straighter line.

Now if anyone had any ideas where to start to look, so I could try to
heat up components carefully, to see which component that is failing. I
woild be grateful.

Best regards Tomas.