Firstly let's eliminate bad potentiometer to complex the troubleshooting. Use proper contact cleaner to blow the Level potentiometer VR340, VR320 and VR321 then; [some people here may scream you not to do that, in that case you can ignore what I say]
You need to confirm below can be done before we suspect faulty components;
Auto Adjustment [Peak]:
Set CH 1 attenuator to 10mV/div,
input 50mV @50 kHz sine (5 div display height).
then
Set attenuator to 0.1 V/div (5 mm display height).
Adj. VR320 (LEVEL button depressed) for just triggering. Then set attenuator to 0.2 V/div, confirm that no triggering is possible.
Depress SLOPEĀ± button, set attenuator to 0.1 V/div.
Adj. VR321 for same trigger threshold. Repeat triggering adjustment as above.
Can you achieve that?