rpm fluctuations
 

my 1999 tiptronic boxster starts up fine and the rpm is steady at 800-900 when i put it on D and my foot is still on the brakes the rpm drops to 600 and the engine starts making a loud noise, i put in back to N or P the rpm jumps to nearly 2,000 and gradually sets back to 800-900.
this happens every time and in addition when going downhill in any gear if the rpm is between 1,000 and 1,500 the car starts to accelerate for 2000 rpms and stop and then accelerate and stop until i put my foot on the gas and raise the rpm higher than that value it starts to work fine.
any idea what could be causing this?

I changed the air-oil separator yesterday and the MAF sensor and both didn't help.

Wouldn't hurt to clean the throttle body...

What kind of noise do you hear when going into D?

Apparently I cannot... "Find the domain of function f defined by f(x)=sqrt(x2+1)+2, where sqrt is the square root." even WITH the formula :D BUT I can tell you that cleaning your ENTIRE INTAKE system, especially the Throttle Body, but even down into and including the intake manifold (use a lot of paper towels) is the best place to start and 'relatively' easy to do.