Worked nice for many years.

Stored vertical for more than one year, inside home, air conditioned
temperature and humidity.

Power up, after 10 minutes the horizontal trace started to present
some anomalies from the middle to the right, like a non smooth
trace.

After more 10 minutes the horizontal trace is collapsing to 50% of its
size and sometimes it goes to the center of the screen, like when
selecting X-Y at the Sec/Div selector.

Sometimes moving the time or changing the trigger mode, the trace
comes back to normal, for few seconds later return to short unstable
horizontal.

I downloaded the service manual and will look for something bad at the
horizontal section, like bad capacitors or something.

I didn't test the X-Y function yet, it would isolate the horizontal
swep oscillator, right?
It seems to be the horizontal amplitude of the scan, I don't think the
fault is at the horizontal oscillator, it seems more to be at the
horizontal amplifiers, its feeding voltage or something like that.

Any hint will be appreciated. I know electronics for long time, but
will be the first time to fix an oscilloscope.