I'm guessing that engine speed at startup is less than what it should be - causing the computer to recognize that something is not right. I've seen it happen on our cars.

The reason could be any of various issues. At this age these cars need someone to go over things like corroded fuse terminals and contacts, split and hardened vacuum hose ends, in addition to a tuneup every now and then.

I like tuneups every 15-20K miles. I know the book says longer, I think 30K, but I just don't buy it. Tuneup = spark plugs, distributor cap + rotor, total maybe $20-25. Get basic copper spark plugs, not the whiz-bang platinums that only work right on newer cars. Bosch parts for all, or NGK for plugs if you like.

We can point you to other things to check. Are you going to get your hands dirty, or do you have a friend who will? If not, you're going to need some money, and a shop that knows and likes old Volvos. They need to get looked over now and then. They often will run a long time without attention but won't do that forever. With attention they seem to last almost forever - the ones in the junkyard usually were still running, only sent there due to collision or the owner simply got tired (or broke) of doing little fixit jobs on a 20 year old car.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.