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