I fixed it!

The problem was the cable. The Radio Shack cable is no good as a trigger cable. The cable that came with my PF60 worked perfectly.

I saw in this post mention of the Radio Shack cable I was using, so I assumed it was the proper choice. As it turns out, it doesn't work. It's an attenuating cable, which explains my voltmeter readings when using it, and also why the trigger signal was insufficient once it got to the other end of the cable.

Thank you brubacca for mentioning that you used the cable that came with the PF60. That made me check my PF60 box to get that cable. Once I hooked that cable up to the 990, my voltmeter readings were a full 12V as expected, and the PF60 was happy to trigger off that.

So beware all who tread after me: The Radio Shack cable won't work!

Lastly, perhaps someone can edit the other post that suggests the Radio Shack cable as a trigger cable.