Sluggish.....

hey guys,
i have an 82rx7 with a 12A and the car runs strong through 1st, 2nd and 3rd when i give it.....but when it hits 4th and 5th.....the car becomes sluggish and starts "bogging"....i think it may be fuel related, could it be the carburator?.......sometimes i have to pull over and sit there for 5 mins and then the car is fine....until i give it hard acceleration again...and when i drive it easy i have no problems...any ideas?
and also, im thinking of having the engine rebuilt or getting another one....are my options limited for my year?
thanks

well the only thing i can think of at the top of my head is maybe one of your roters are skiping, also how high can you rev in 1st , 2nd , 3rd.