I had problems with 16T and 18T on my 7-speed and 6-speed cassettes, back when I used to have a powerchair motor driving thru CrazyBike2's regular bike drivetrain. This was with regular narrow multispeed chain, rather than BMX, but I expect BMX would've done the same thing. My problems were at startup from a stop, but if I had been trying to get power enough thru it for high speeds it probably would've done the same thing.
My eventual solution was to change the gearing upstream for larger rings, so that I had more teeth on the rear cassette gear I used for the same "top speed", and avoided using the 16T and 18T (because the skips had damaged their teeth! Even now, I can't use those two sprockets even for pedal-only or it skips).
Another thing that would help is increasing chainwrap, so more teeth are engaged, but I don't know if you have room for that on there. You'd probably have to build a tensioner specifically for that, or order the one NuVinci sells if they still have it.
EDIT: Also, I thought I'd point out my solution to FS and chain tension is to use the swingarm pivot as the chainring center, so there is no growth/shrink of chainline during bump/droop.
In my case, the pivot is actually the NV, and the wheel will just have a chainring, but the reverse would work just as well.
You may not have that option with your setup, without frame modifications, though.