I think paper strips and IPA is the most-recommended way of cleaning those contacts. I have also followed up with a dab of Deoxit with positive results. (Some might disagree with that.) Just go slow and be careful not to bend the leafs. In addition to the ones you can readily access by taking off the shields, there's some in the back too, and that requires disassembly (not fun.) Pull out the attenuator modules (10X and 100X, if memory serves) and clean the pins on those as well. That also gives better access to the contacts. Careful not to swap the modules between channels.
There is also a leaf switch at the back of the vertical amp board for Trig View and BW Limit. If one side of that gets flaky, it cuts deflection in half. That's pretty easy to get to, so clean it first, it may solve your problems.