I have a 1989 Saab 900 Turbo. When the weather is hot (over 90) and I drive in stop-and-go my clutch gradually weakens until it is all the way down to the floor. If I leave the Saab overnight and it is cool, the clutch restores itself fully. This low-clutch problem never happens in cold weather. The reservoir is full. Could it be an air bubble that expands in hot weather? Could it be a problem with the brake/clutch fluid?
The fluid can break down with age so you could try flushing the old fluid out. For some odd reason this can sometimes be a pit of a pain on a SAAB.
This could also be caused by a failing clutch master or slave cylinder; or both. Failures of these 2 parts are not uncommon with age.
Technically, when one is replaced the other should be also. Changing the slave cylinder is a bit more complex so if the fluid flush does not accomplish anything you could consider the clutch master cyl. only.
If the slave cylinder is replaced this would be a good time to change the clutch assembly also. You do not state if you do your own work or not but be aware that servicing the clutch slave cylinder or clutch assembly on a SAAB like this involves a few odd techniques.
If it gets to the point where the clutch pedal is gone permanently the slave cylinder job will become much tougher to do. Hope that helps.