i picked up the 7 about a week ago and i love the car, everythings stock, the only thing is when i take it up pass 5 to 6 its starts makin a humming noise and it starts to shake. i was told it could be a series of things like the drive shaft or the tranny mount, now im still new to the whole rotary scene but im very eager to learn everything and anything, now i know it takes time but when your speedometer redlines around 8 something and you can only rev it up to 5 or 6 that really sucks.
please help...

also will i have any problems replacing the stock radiator with an 88 turbo one?
aside from the filler cap being in a different place and it being about 3/4 of an inch longer in lenght...

 

that could be a lot of different things man, it could even be something as simple as bad tires, front end alignment, wheels out of balance... any of that could cause a shake at higher speeds.

 

So the shake is RPM dependant? Check to see if your fan is chipped or broken.

No problem swapping rads.

 

the tires and alignment are fine and yes i would say the shake is rpm dependent cuz it shakes on every gear whenever i take it up 5-6.

 

The the first thing you should check for is a damaged fan, bad engine mounts, or bad transmission mounts.

 

when i attempted to changed my radiator i did notice that the fan is chipped like it had been grinding on something i dont know what. i was told to replace it with a fiero fan which would be far more suitable.

 

The Fiero fan isn't "more suitable", it's just cheaper and easier to find. If you are going to go the electric fan route, please read http://www.aaroncake.net/rx-7/efanmyth.htm and http://www.aaroncake.net/rx-7/efaninstall.htm