Bent transmission output shaft?

[Zhanshi]

I have a 93 240sx, it was originally auto, but was swapped to manual last week. As soon as everything was installed I started the car. After shifting to first gear, I noticed that the shifter would start to rotate in a circle (counter clockwise). This only happens at low rpm and starts to diminish at higher rpm. Is it possible that the output shaft is bent? Since at higher rpms it reaches a sort of "ressonance frequency" so the rotating shifter isn't as violent? The gears 2 through 5 and reverse shift smoothly, but at times, first gear requires some force to engage. I tried searching, but came across no symptoms of a bent shaft. Thank you in advance!

[rotorimp]

Try bleeding the clutch master/slave cylinder. Have you changed the trans oil? Does it rotate in all gears---Is the shifter installed properly?

[Zhanshi]

Just did the swap, so the clutch lines are bled and theres new trans oil (about 2.4 quarts). The shifter should be installed correctly. As for the rotating, it only happens at low rpms and it seems as if the whole transmission rotates too. Any ideas on whats wrong? Thanks

[rotorimp]

What condition are the motormounts in? Is the trans mount secured? What weight oil did you use?

[Zhanshi]

Mounts are in ok condition (no problems with auto trans), the crossmember is torqued down with mount. I used redline MTL (70W-80 GL-4).