First, make sure that you have the correct spark plugs and they're correctly gapped. I strongly recommend that as a first step, you get a set of the stock NGK plugs from a Saab dealer. For the '91 and later Turbo: NGK BCPR7ES-11, Saab part # 02 44 459. For the '91 and later naturally aspirated: NGK BCPR6ES-11, Saab part # 02 44 772. The correct gap for both is .039in. Gap them carefully!
Another thing to do before you put in new plugs is to look very carefully at the insulators of the old plugs. If dielectric grease hasn't been used on the plug boots in the past, the spark can and will arc down the outside of the insulator to the base of the plug. The evidence is a faint black (probably discontinuous) line on the insulator. Every time you remove the DI unit, apply dielectric grease to the plug boots. Get some dielectric grease (at any auto parts store) and apply a light to moderate coating to the inside of the plug boots before you reinstall the DI (make sure the boots are clean and dry first). I cannot emphasize this too strongly (it cost me a DI once). Inspect the DI cassette while you're in there (look for cracked plug housings/oil leaks). Reassemble and see if the problem goes away. If so, you're out about $13 and you have new plugs and good, water tight connections.
If this doesn't fix it, the DI cassette is the next suspect (assuming that you have a car with Direct Ignition). It's somewhat more expensive though ($300)! There is some debate as to whether the DI cassette design was ever improved; some say it was upgraded in '94, others say it was not. To protect the DI, use the correct plugs, keep them properly gapped and use dielectric grease on the boots; in short, do everything possible to keep the engine from missing.