How to diagnose insertion point/trackpad jitters?

When I’m typing in any application (e.g. Chrome, Safari, MS Word, Evernote), the insertion point often jumps around unexpectedly. I have this problem with the builtin trackpad and my magic trackpad at home. Caveat Since this is both a disease of the internal trackpad and the magic trackpad then it can't be the cable between the internal trackpad and the motherboard

My MBP has been for service once, I believe they changed a cable between the trackpad and motherboard. No dice. I have wiped the OS and reinstalled from scratch. I have booted in safe mode. You get the idea. I'm certain that most of the time I'm nowhere near the trackpad when it happens. (Like 10 seconds ago).

Is this hardware? Software? I must admit I don’t have a clue how to diagnose. My leaning is likely software.

MacBook Pro (Retina, 13-inch, Early 2015)
OS 10.12.5 (although this has been going on for over a year)

I'm sure you've read Apple's support documentation on this issue by now and have had no luck.

Since the issue wasn't resolved even after reinstalling macOS from scratch and booting into safe mode, this should definitely be a hardware problem. Your trackpad is slowly dying. The connector is probably loose or getting old. Since you say that the trackpad cable has already been changed once, it probably wasn't serviced properly the first time.

The best thing to do here would be to go back to an Apple service center and get your laptop checked again. That's almost always the best route to take in case of hardware problems with Apple products.

FYI: There are people who've said that a firm slap with the palm of their hand fixed their trackpad issues. Try at your own risk though.