It is all so complicated to look component by component, but a swap-in of a working ECU will instantly correct everything if that is your problem. That is the way I would diagnose it, with an ECU swap and an AMM swap right off the bat. Look for a rebuilt or certified 561 or a 951. They are often marked "Testad" and "CK CK" I think.
I have an '89 and a '91and have two spare ECUs to avoid these types of episodes. At times there have been reports of faulty manufacturing on these made in Mexico units and reports of connector pins separating internally. Also, there is one guy on the list who knows of a Volvo upgrade kit that repairs the condition. As for me, for a pittance, I get a unit from the junkyard and test it immediately. Then it stays on the shelf for a rainy day. Most I have found are good, rebuilt or original 561 or 951, but reports of failed units are common enough in '89 240s and 740s. Many cars I see have the rebuilds or upgrades in them.