Hi,

You stated that you went to Napa and got a new distributor cap. You did not mention whether you got the "rotor button" that spins under that cap.
They go bad by at first having problemslike this because the resistor, that is buried down inside a black or brown epoxy gritty strip, turns bad.
The strip runs between a brass center pad and a brass on a contact tip. Sometimes an ohmmeter can detect one being bad but replacing or keep a fresh one in there is best. 70k or several years might be my limit as I only scrap the green off the cap terminals so many times as there becomes a divot in them.

As the rotor gets worse the top will eventually crack and burn open. At this time the car will no longer start or really run rough.

We don't know if there was a previous owner before you or not and how old the ignition wiring system is. Especially the main coil wire can be an issue. Lots of wire set refuse to include a new coil wire! Moisture truly affects ignition!

These ignition and fuel pumps should all Bosch components upon replacement because they are more reliable but do not last forever.
Since the early VW bugs they have used this same distribution system. They are very reasonably priced. Some Brickster's like Bouricord spark wires over other wire sets. They seem hard to get in chain stores.
I swipe what ever looks new on junkyard cars when I'm lucky to get them. My sources are drying up.

Always carry a spare little bag of other goodies. There is a list of things you can put together are recommended by Art's Cleanflametrap.com website for roadside emergency troubleshooting.
If you follow his site or BB posts you can learn how to almost always stay out of possible tow.

Napa may try to pawn off another brand like Standard Auto Parts. Their products are made by who knows who. They are alright in a pinch.
Another brand they like is Niehoff.
They are considered an American top of the line manufacturer of domestic parts so I don't necessarily feel bad about having to use their parts.

The general consensus on the board is that Volvo picked the best parts for their money value and reliability. All of us who have tried aftermarket products usually get disappointed in one way or another OR sooner or later put back original.
With that said, I will not say there are not better products or technologies out there. It's just some seem overpriced, for me, to get all wide eyed about.

Not having dash lights when the motor dies is a big clue!
They should even flicker on and off as it dies!
Did you ever check the fuse panel for corroded or loose fuses.
Sometimes a windshield leak hits them or some relays located elsewhere.
Feel under the carpets and into the padding, on both sides of the car, in case water is getting by you some place.
Have you ever cleaned out the rocker panels from leaves? This would be a good time to check out both.
Water vapor will rise and fall and settle on those fuses, as the hinge post is a cold, no air circulated place. Water coming off our shoes during a rain goes there as well.
I keep a lightly sprayed on film of LPS ONE, once a year, on mine!
Winter is coming of which this problem is going to get worse.

TLC goes a long ways when compared to being dead on the road in bad weather.
Phil