help me please
 

k. i have always liked mazda rx7s and dont see any around here. i finally got one. its a 1988 mazda rx7 se. it has 117000 miles on it and i paid 500 bucks for it. problem is the back two plugs are not getting fire, and in order for the car to keep gaining speed i must shift around 6500 to 7000 rpms. if not they slowly go down and so does the speed. i have asked several people and nobody knows wut is wrong. possibly back rotor? if so where can i get cheap parts? i am on a tight budget due to saving for lincoln tech where i want to do paint and collision. please can sumone help me!?! please and thanx.
-NICk