another riddle my 240 gave me

[Russki]

Help me figure this one out:

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, sometimes worse than others. Once in a while it doesnt do it at all 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 anybody has any suggestions, please help me out because I'm stumped.
Thanks.

---Gennadiy

[tracer bullet]

So, is it just speed dependent - does it do it in 4th as well at that same speed?

Is it RPM dependent - does it happen at the same RPM cruising at a lower speed in say 4th gear?

If it's only 5th gear... I dunno, 'cause I don't know much about transmissions. But I think that's where you look next.

[Russki]

It's neither... or both, depending on how you look at it. It doesnt do it in any other gear at that rpm, and it doesnt do it at any other speed either. If i'm in 4th at 65-75mph it doesnt do it. If i'm around 3,000 rpm in any other gear or in neutral it doesnt do it either.

The vibration is coming from the engine so I don't think that the tranny is at fault. I was thinking maybe some type of a vaccuum problem but I have no idea where to start. The tech at work doesnt know either. I'll try replacing the cap, rotor and wires next, see what happens. Maybe even hook it up to a nissan CONSULT computer, dig around in there a bit.

Thanks for the reply. If you have any other ideas please share.

---Gennadiy