Well, now I feel like a bit of a dummy.
Turns out the reason for the sporadic audio had everything to do with the tracking adjustment. I poked around with some of the stuff on the front panel and apparently you can use the level indicator to adjust hi-fi tracking for your tapes. A very helpful feature!
I also found a service manual, but I'll end up needing a neck brace after I read through it because the whole .pdf is sideways. Once I get a new toner cartridge for my printer I'll see about printing all 149 pages.
Now all I have to do is replace some rubber and I think it'll be ready to keep on playing.