Stock amplified speakers cutting out in Volvo s80

The most likely cause is the deck (because it is the simplest scenario).

In some cases the factory amp can also give feed back and shut down the deck.

No matter the case the best bet would be to replace both the deck and factory amp with better equipment.. some of the aftermarket replacements will cost nearly as much as replacing the stock equipment.

You would of course want to replace the speakers in that scenario as well.

The factory stock decks aren't worth getting inspected fixed or otherwise.

You could probably wire a pioneer 80 to the stock speakers if you had a suitable electrical diagram for the car as well... simplest solution.

you should just bypass the stock amp and connect the speakers directly to the deck.