Hi,
I had the same symptom about 15 years back, with a 1983 Saab 99GL (2,0 H-engine, single carb). A real surge of bubbles in the coolant when taking the cap off, too. Almost burnt hands and face. The pressure eventually cracked the reservoir tank. There was also a risk that a coolant hose would fall off due to the pressure.
I got it home after 300 km *very* careful driving. When I pulled it all apart, I found a crack beside the exhaust port of no. 3 cylinder. Exhaust gases were reaching the coolant, and coolant would seep into the cylinder, which meant it started only on three cylinders for the first couple of minutes.
The integrity of the head gasket was good though, everywhere.
Luckily I got a head from a wrecker's (from a 1982 C900). That plus a new head gasket and 15 hours of back-breaking work (I was new to this kind of work :-) got me back in business. The car lasted a further 11 years; it "died" in 2000 due to body rust. I traced the problem back to a day when the cooling fan had failed to come on, causing overheating. Saab heads have been very sensitive to temperature; a non-functional fan increases risk for a cracked head.
This (replacing gasket and/or head) is not a job that a Saab mech must do; any good mechanic whom you trust can do it. Ask to see the head and gasket once they're off. Check the head very carefully for cracks; a hairline crack is sufficient for causing the problem you have. See if the gasket has been breached, too. But make sure to get Saab's recommendation on head bolt torques, tightening sequence, etc.
Two more hints:
Good luck, I hope it's just the head gasket. Tell us how it goes.
/Robert
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