I that's the Volvo engined car that's the Facellia. Sweet, and rare.
If you get a burst of white smoke from the exhaust when coming off the gas when running down hill that's almost certainly brake fluid burning off. It leaks into the servo and then get sucked into the inlet manifold. Servo will need repair or replacement. If you've been topping up the brake fluid a lot, check if you haven't as it may be all gone, that's also a pointer. If you loosen the servo from it's mount and give it a shake you'll likely hear fluid sloshing around.

I assume you got in your trunk? See other thread.