What causes the car to lose power once it reaches 40 mph?

Hello, test equipment is needed to find out what is happening by checking data from the engine's processor. One guess is that you may have an exhaust restriction choking off engine performance. But this is only a guess. You could even have an automatic transmission concern. https://repairpal.com/repair-shops/volvo-repair-and-service-in-salt-lake-city-utah