I got a code of "misfire in cylinder 3" so I changed the plugs (fine) and distributor cap (pitted) and thought that would do it, since it ran like a charm after that, for about a week. Then, while towing my boat across an interstate bridge it quit and would not start again. I had it towed home, tried to start it a few hours later and it acted like nothing was wrong. 
I drove it around the block, turned it off and on over and over again, no problem. So, I decided to drive it the 16 miles to the dealer instead of having it towed again - and it quit a few miles from my house. And when it stopped, it was like I had turned off the key, except the dash lights were on. What I mean is, it didn't struggle or misfire - it just stopped running. Turned out that both the brain and fuel pump had gone out. $1200 but worth it to trust the car again. The dealer found a used brain and an after market fuel pump - that is the first time a Volvo dealer had ever done that for me. I appreciated it.