compression pumping effest on mixdown

I am using a standard Dell laptop with Windows 7 and Cubase 6 64bit
The sound I get when I mixdown is inferior quality to the project when I play it in Cubase and there is some sort of pumping/tremelo effect going on in the background.
I have tried Real Time Export but it keeps saying CPU overload no matter how few plugins I use and I have tried all the usual things like checking nothing is going on on the master bus and making sure I have dithered 22uv etc.
Anyone got a suggestion?
It’s very frustrating!