Remember, a keep-alive isn't going to do bupkis if the engine is losing contact permanently, which is what a stall usually means. If it loses for a fraction of a second and stalls, and then goes through its restart and continues on, I could see a keep-alive helping. But that's not what this sounds like.
I'm not a fan of those plated metal pickup strips in tender trucks. That sounds like what's in the Bachmann tenders and they don't work as well as the plain phosphor bronze ones in a Kato tender. In any event, yeah, clean the cups and see what happens.
There are more hard-core things you can do to positively prove that it's a pickup issue. It all depends on how deep you can dig. I have taken a spare Kato tender coupled behind the main tender, with jumper wires across to the feeds in the main tender. If it stalls after that, it definitely isn't in the wheel or pickup system; it's probably in the wiring or drawbar connections.