Note: Pre-90 normally aspirated and pre-91 turbocharged cars do not have the DI cassette.

A notorious Saab 9000 failure item.  DI stands for Direct Ignition.  Cassette is a fancy word for "large module".  It's the bright red thing that sits right on top of the engine; you can't miss it.  How it works.  To replace it with a new item, you take about three hundred dollars out of your wallet and exchange it for a new DI Cassette.  Then you open the hood, remove one connector and four bolts, carefully lift the old one straight up and plop the new one straight down onto the spark plugs; re-install the four bolts and the connector and close the hood.  Takes about fifteen minutes.  And three hundred dollars.  For a used one, the procedure is half to two-thirds as much, but you don't know what you've got.  People say they've gotten rebuilt units, but in all but the earliest units the components are sealed in epoxy, so there is no rebuild option.  It all sounds like a rotten deal, but at least you don't have to worry with distributor seals/bushings/whatnot, and plug wires and such.

Symptoms:  Engine misses under load/when the turbo is boosting; may also be related to temperature or damp conditions.  Any time your engine develops a miss, change the spark plugs first.  Saab 9000 turbos are very sensitive to plug problems.  It's not that they eat plugs, they're just not tolerant of  the wrong plug or an incorrectly gapped plug.  Use the stock NGK plug, and gap it correctly.  If the plugs don't fix a missing-under-load problem, the DI Cassette may be at fault.  Generally the condition will continue to worsen over a period of weeks.  I can say from experience that the DI unit can fail totally and without notice.

Preventive maintenance:  Do not allow the engine to miss.  Use the stock Saab plugs, keep them gapped correctly and correct any engine missing immediately.  Use dielectric grease on the spark plug boots (this is VERY important, as the spark will travel down the outside of the plug body and cause a very bad engine miss if you don't use the grease).  I firmly believe that if you do the right things and keep the engine from missing, you won't have the typical DI cassette failures!  I figured out in retrospect that the lack of dielectric grease caused my engine to miss for some period of time, and I firmly believe that it caused the DI cassette to fail.