vibrations @ about 2000rpms? Help!

I have research this to no end on the internet and seems to be about 500 reasons..i hope some one here may be able to help.
I had a 2000 vehicross motor put in my 99 vehicross, the motor only had 50,000 miles on it ,it runs strong and well with the exception that sitting still or driving it starts to vibrates around 2000 rpms and then goes away after about 2800-3000rpms, i thought motor mounts ,so change them out ,still the same,change the transmission mount ,still there, i would think harmonic balancer but,i notice if i park it on a really steep hill with the nose pointing down hill...and try it the vib is gone?? what the F_ _ _! any other postion it is there..any help? im going crazy trying to find this!

You may have a coil or plug that's going intermittent due to the engine frequency at that RPM causing it to temporarily fail. Is it a hard vibration, like missing on a cylinder?

It is not a real hard vib, its more like a rough idle vib but of course at higher rpms...im not really sure if it goes away after 3000 rpms or just the frenquency is so fast that it feels like it goes away, kinda like a washamachine when you first start a spin cycle and as it gets faster it kinda seems to smooth out..If it was a coil or plug, having the vehicross on a steep hill pointing down should not make it go away would it?