Und nochmal was.
The cassette is held onto the engine with four bolts. Typically 10 mm. Just remove the bolts, and lift the DI Up. Lifting one side first a little helps, and the DI has a nice hand grip built right in.
Newer DI's have a cable that disconnects at the DI. Don't bother - you can do this with the DI right at the engine.
Flip it over - you'll see a plastic cover, which has four cylinders rising up. It's held onto the DI with some Torx screws. Undo the screws, and simply lift the plastic cover off.
You'll see four whitish cylinders (coils) sitting on the circuit board. In older units, you can see components on the circuit board - on newer ones, the board is totally potted. The coils can be lifted off. Don't. Nothing bad happens if you do, but just make sure you put it back correctly.
I hold the DI up to a bright light, and look for the oil level in the coil. It should be around 90% full. This isn't an exact measurement. Good coils have oil, with just a little airspace showing. Bad coils have very little oil left, and have signs of oil leakage. You may also see burning/carbon buildup on the metal tip of the coil.
The coils can't be serviced - you can't add oil. The coils are removeable, but I don't know of anyone replacing just a single coil.
There really isn't any maintenance. If you see burning/carbon on the tip of the coil, that indicates that the spring (still in the rubber boot on the plastic bit you removed) isn't making good contact. I'd brighten up the surface and check out the spring. However, my guess is that if this has been going on for long, the DI is on the way out.
Put things back together by putting the cover back on and tightening (not overtightening!) the screws. Make sure the springs haven't fallen out (don't ask me how I know this).
Pull the spark plugs and check the gap. It should be 0.040 (1 mm) If the plugs look worn in any way, replace them. Cheap insurance. If I need to re-gap them, I always go for 1 mm, but cheat on the narrow side, as plug gap usually increases with age and use. I stick anti-sieze compound on the plug threads; WD-40 will work, too. Makes them go in (and more importantly, come out) easier.
Put the DI back on and tighten the 10 mm bolts.
The DI either works or it doesn't, and frankly, requires maintenance like a light bulb - replace it when it doesn't work. Since a dead DI means an engine that won't run, it's nice to be able to avoid some percentage of failures that will leave you on the side of the road. Inspecting the DI regularly should give you some warning, allowing you to get a new one. And by checking the plug gap regularly, you will keep the DI running longer. This is not a perfect system; there are still DI failures that occur with no warning (zero prognostic lead time, in the parlance). But reducing risk is still good.