need advice on my 240 plz

[sleepingpanda93]

ok got an 89 240sx when im driving on the freeway when im in 4th gear or 5th gear i lose power as soon as i hit 3500 rpm and by lose power its like i popped it into neutral and my rpm's begine to go down, the car doesnt turn off or anything but it does stop letting my go any higher, already checked my maf and for awile it did like me go to about 4500 or so but i didnt need to keep going and next day lost power again at 3500 so idk wat to do now.. help?