A few months ago, I installed Kooks headers, Yank SS3200 stall torque converter, and an FLT (Level 3) auto tranny to a standard rubber tranny mount. Just prior to that, the only other things I've done is add a Meziere electric pump, ignition wires, plugs, MSD cap/rotor, new Yokohama tires all around.
A couple of months ago, I replaced the broken stock rubber tranny mount with a poly mount and noticed an increase in vibration. But in the last month, or say, 1,000 miles, I've noticed a fairly "violent" vibration. I've rotated the driveshaft 180 degrees with no improvement.
(yes, I realize that more vibrations are expected with a poly tranny mount, but these vibrations are out of the ordinary).
The vibration occurs mostly between 1,800-2,000 RPM, and seems to be most apparent in 3rd gear - if i manually shift through the gears, it's not so noticeable in 1st or 2nd, but is most noticeable when i shift into D. In fourth, (circle D), it's not as apparent.
I have also noticed one other thing. Let's say I'm running at 50+ MPH and above 1500 RPM and I let off the gas. When the RPM's hit 1500, the tranny downshifts to 3rd VERY noticeably (never did this before). Also, if I mash on the gas (say, from a stop) conservatively, the tranny shifts into overdrive prematurely, at around 1500 RPM at 45 MPH, which it never did before.
So here's my question - and I'm mostly interested in determining the vibration problem now. What is the best method to determine where the vibration is coming from? Could it be the flywheel, harmonic balancer, torque converter, tranny, or driveshaft components? How can I reliably eliminate where the problem could be coming from?