It's mechanical - the doughnut slides off, especially if it's used upside-down a lot (i.e. in a router table), as gravity becomes an accomplice in the crime.
I documented all this with my Trend T11 use the search function of the forum to find the pictures of what you're up against.
The electrics on the T11 are 'potted' meaning there's thick plastic so you can't get at individual components The Elu might be the same, or it might be bare components (it being older 'n'all). The disc with the magnets in it on the shaft, needs to be glued back so that the centre line of the disk (its thickness) comes to roughly the centre of the potted circuit board (again its thickness). That will mean the strongest signal out of the tacho and best reliability in the future. You may see a mark on the shaft of the motor where it was originally, correctly fitted.
If it''s not potted components and there's no visible mark on the shaft, the tacho sensor will be an obvious component on the circuit board - probably a rectangular black plastic block that comes quite close to the circumference of the disk on the shaft. Set the disk so that its centreline is roughly aligned with the middle of the block (height-wise off the circuit board).
There is a bit of leeway with this - the alignment doesn't have to be perfect. Mine didn't start misbehaving until the disk had almost completely come off the end of the shaft.
I would strongly recommend using proper thread locking compound from a motor factors (auto spares shop), and NOT superglue or epoxy resin. This is because you may one day want to replace the main bearings, which are behind that magnet ring, and you need the shaft to be clean to do that.
Proper threadlock fluids are designed to grip tight, but can still be cleaned off the shaft reasonably easily. Neither superglue nor epoxy resin (Araldite) are easy to remove in the same way. I use Loctite 243 "Lock 'n seal", as I've had a tube of it for years, but anything similar will do. Thread locking compounds usually take an hour or so to go off properly, unlike superglue: you can blot off any excess with a bit of kitchen towel, but don't spin up the motor when the stuff is wet or it will be thrown around inside the casing -- you usually only need a very small amount, and it's designed to creep into threads and tiny gaps.
I do hope that's what's wrong as it's a very obvious fault and an easy fix. Don't forget to remove the mains when you open the back cover, as you expose a lot of bare, possibly live, bits.
E.
