Engine Swap Help

[mbC230]

I have a 1997 C230 on which I performed an engine swap. I put everything up, started the car up and it idles fine. But when I accelerate the rpms go up to about 1500 and then drop down and goes back up. The engine keeps osciallting like this until I turn of the car and try again. Again it idles great but when i hit the gas the same thing happens. The car is in park when i do this. I didn't have this problem with the old engine. Could you guys tell me whats wrong and how i could fix it? Thanks