Vibrations

I have a strange thing happening on my Vette (92 LT1):
At a certain RPM (around 2000 RPM) I can feel vibrations that seem to come from the engine, and I have the impression of a loss of power too.
I thought it was the tranny first, I then had it checked, and it looks fine...
I then realised that my spark plugs were to be changed, I repaced them all, but after that the problem remained. I still have those vibs coming...
What do you think guys it may be (cause I am out of idea now
)Thx


