V50 Engine/tranny Problem?

[V50]

I am looking for help on my 2005 V50, I have already brought it to a Volvo dealer andthye told me there was no problems, but they couldn't drive fast enough to fing it. My V50 is an Auto when I step on the gas the car seem to go but then bogs down and accelerates slowly. Also the engine sounds as though it is screaming but the RPM's show normal. This happen in 4th and 5th gear between 3 and 5 thoudans RPM's. But the catch is that this does not happen 100% of the time. Sometimes I can shut the car off and then the car may or may not return to orginal performance. It's as though it drops to 4 cylinders when this occures. My volvo shop says something is wrong but i would like to give them an idea of what it could be so I don't spend alot of money have them rip the car down to find the problem. It is reporting no Engine or Transmission codes.

Any Help would be great

Thanks