Welcome to the Forum, Bruce.
I'm not an expert, but I've put a lot of time and money into BJ8 speedos (I think I have two 'spares' I acquired for parts in the parts drawer). Speedos are remarkably simple in theory and design, but are akin to a mechanical watch in operation. The angle drive and cable turn a magnet, whose eddy currents cause a disk--to which the needle is attached--to turn at a reduced rate; sort of a first derivative to the angular speed of the cable (like velocity is the first derivative of acceleration). The tripmeter and odometer are operated by ratcheting mechanisms that are directly driven by a worm gear off the cable. They can cause other issues, like a periodically jumpy needle.
There are a couple of things I can think of that would cause your problem; the first is that the needle is dragging on the speedo face or its 'bearing' has worn; have you tried the old movie trick of tapping the glass when you have decelerated or stopped? If that causes the needle to move, then that's your problem (the needle is 'returned to zero' by a tiny, weak spiral watch spring). The second is something else sticking inside the speedo. I don't think lubrication--I recommend a silicone lubricant as they seem to attract less dust--will fix your particular problem. If it's not the needle dragging you have to open up the speedo; something that is not for the faint of heart, but if you do there is some info here:
https://triumph.daveola.com/NOTES/Speedo_Repair.pdf
NOTE: The angle drive needs a washer:
https://www.mossmotors.com/graphics/products/PDF/021-511.pdf