These behaviors occur randomly when I'm writing in text editors like Text Edit and word processors like LibreOffice (and possibly hybrids like Bean, though I haven't used it frequently enough lately), in online text boxes, and when I'm composing mail -- just about anytime I have to write.
As a professional writer, curbing this behavior is vital to me.
The cursor doesn't bounce around, but the insertion of characters is unpredictable. I believe the problem is worse on some websites than others -- on many, like Communities, the problem doesn't occur at .... WHOOPS. It just happened here. I haven't kept close track, but my sense is that the problem is not restricted to a certain set of characters. And it involves not-alphanumeric inputs, also.
I don't know if this is a hardware or software problem, as it's been happening on successive Macs at least since OSX came out; and now on my Macbook Pro (mid-2012) running macOS 10.14 Beta. Each new OS should have cured a software problem, if there was one. And each new Mac (at least since my first modern Macbook Pro), with its new CPUs and a new keyboard, should have cured a hardware problem. But still the problem persists.
Thanks for your suggestions for remedying this problem. If you or someone you know has it, that would at least confirm that it's not just a one in a million phenomenon, Apple's big love for me as a lifelong customer.