Problem With 2005 Volvo V50 Turbo Temporarily Losing Power When Going 50mph or More?

I am uming that your independent mechanic checked for any diagnostic codes that should be stored in the cars computer (ECU). If he didn't, the next time it happens you should check the date and time and then those codes could be pulled to see what was happening at that time to cause your problem. Also, note whether you had your automatic climate control on. However, from the sound of it, you could have a number of issues. My guess would be a bad coil on one of your cylinders or a wire to one or more cylinders that is going bad (probably one near your turbo) due to exposure to heat. Also, if had your climate control on, maybe the A/C compressor is not disengaging properly and that would cause the lag you experience. Another possibility is bad ECU itself, but then you would be experiencing other issues as well. I also suggest you check the Volvo forums and see if anyone else has experienced a similar problem.