Had the same problem on my '89 turbo. Turned out to be the ignition wires. I had replaced them also but purchased a cheaper "standard connection" brand that was designed to make contact with the walls of the distributor socket holes. The Volvo distributor has posts in the holes as contacts. The standard wires never touch the posts and creat an additional gap that the spark voltage has to jump before it reaches the plugs. This weakens the spark and causes missfires. If you don't have the proper wires I would suggest replacement.
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