Engine vibration only under certain circumstances

[krakadil]

I have a mostly stock '93 hatchback with 136,xxx miles

When in 5th gear, at 65-75mph only while accelerating or decelerating I get a vibration from the engine. It feels exactly like if you're in high gear at too low of speed and pressing on the gas, and the engine kinda struggles to get up to higher rpms.
It happens almost every time I'm at this speed. Once in a while it doesnt do that but I can't figure out what makes it stop. I have a K&N cone intake with polished aluminum pipe, but the problem was there before I had it and hasnt changed since I put it in. I have also replaced the fuel filter with a factory Nissan one, replaced spark plugs with NGK's, changed the oil, and always have used premium fuel. I have a factory service manual but could not find anything in there that describes my problem. If anyone has experienced anything similar to this, or has any suggestions, please help me out.
Thanks.

---Gennadiy

[Bill Roberts]

Its just a resonant frequency. Mine does it at 2750RPM.

I cured a lot of the problem by going under the car and looking at all the mounts from all the exhaust hangers, transmission mounts and dampner...everything that connects the drive train to the frame. I had a bent transmission dampner, I replaced it, added another one and the vibration went down seriously.

Look at the drive shaft bolts, make sure you are not missing any. You may have a rotor that is warped or another feature I am overlooking.
Just get it on a lift and look at EVERYTHING. Anything that can transmit the drive line vibration to the frame and or body can make this happen.

Good Luck!

[krakadil]

I have thought of engine mounts and driveshaft being unbalanced but that would make a vibration only during certain rpm's without depending on the speed of the vehicle. My problem only occurs at 65-75mph in 5th gear around 3,000 rpms. If I am in 3rd or 4th gear at that speed it doesnt do it. If I'm going any other speed at 3k rpms it doesnt do it either. It also doesnt matter if A/C is on or off.
I have done the z brake swap for front wheels and put on new rotors/pads on rears but the problem was there before the swap and hasnt changed since.
One of my guesses is maybe some type of a problem with the vacuum system? Maybe some valve is sticking or a line clogging up, but I have no idea where to start or what would cause it.
I have been wondering about this one for months, so I come here for advice. I appreciate any responces.
Thanks

---Gennadiy