fuel pump or injectors?

[ROBOROB]

well i have a weird problem on my s14. ive had the s13 blacktop in it for about 3 months, but now its starting to sputter and jump when i get on the gas. but the funny thing is, it only does it from full until a half a tank. as soon as my gas gets to a half it runs great. i have no idea what this could be, maybe some of you experts can give me some idea of where to start.

thanks