This may or may not be related.
OP mentions that the issue does not happen on a wired keyboard. Same CPU otherwise?
On laptops, there is a touch pad right where the thumbs like to hover. And occasionally touch, or get close enough that the touch pad picks up the signal and moves the cursor to where ever you left it and the missing letters end up there.
"End up there", could be elsewhere in the text, or if the touch pad (as opposed to the keyboard) cursor was left outside a text area, the AWOL typed text ends up in the ethos of nothingness.
If your situation of concern does not involve a touch pad, then the problem is most certainly gremlins with in the computer that are out to get you.
Dave, that's my story and I'm sticking to it. This happens to me all the time (both).
Oops/Edit: My wife's computer has an older MS wireless keyboard and mouse (no touch pad). First, if there is a lot of junk (books, papers, pens, dust, microns) between the keyboard and the sender-thingy (tech-talk), it doesn't work and text can be lost. B, if the batteries are low. Yep, this is a not so tech tech device. Both require those antiquated products called AA batteries.