#5 Misfire Sometimes

[civic lx]

2002 Olds Sil.

I've been messing with this thing for a year now. I have had a miss on #5 off and on. The injectors were cleaned right after the engine was rebuilt. Plugs and wires were new at that time too. I have since replaced #5 injector, which helped and the CEL went off for a week. I have noticed in the last year anytime the temp was 75 or above the van ran and started great. If below that temp it took just a few seconds longer to crank up and would chug or miss a small amount. Also after the the injector replacement if the fuel level gets below half, it likes to miss on start up. Now with a full tank it will act like it is suposed to. I don't have a way to check the fuel pressure but I was thinking the fuel pump is getting week and causing my problems. What do you think?