Zombie pigman spawning infrequently in gold farm

I have a world that I play using minecraft 1.7.10.

Using creative mode I have built a zombie pigman farm where zombie pigman fall to their deaths after stepping on a pressure plate and being pushed into a trench by a piston. At the bottom of the trench are hoppers chained together so where ever the zombie pigman dies a hopper picks up the dropped loot and sends down the chain until it reaches a collection chest. On top of the hoppers are half slabs. Where I stand is right next to the chest on y = 5.5. The collection chest is directly below and to the side of the trenches so I can hear the zombie pigman dying.

I have modified the environment to the point that no zombie pigman can spawn within 128 blocks of where I stand in the collection area unless they spawn on the spawning pads directly above. The spawning layers are as following: y=35 y=40 y=45 y=50 y=55.

I know that no zombie pigman can spawn outside of the farm because I have removed all nether rack from layers y = 32 to y = 122 inclusive and layer 32 is lava source blocks. I also know that there are no caves in the remaining nether rack layers y < 32 and y > 122.

I copied the world out of the saves folder for minecraft 1.7.10 and pasted it into the saves folder for minecraft 14w32d.

I then launched minecraft and monitored the collection chest. I stopped hearing zombie pigman dying and my chest stopped filling with rotten flesh and gold ingots.

I think that spawning is broken somehow.

Update: after flying around the spawning pads for a few minutes in creative mode with F3 debug information displaying, I did occasionally see one or two zombie pigman. However, my E value in the debug information has only gone up to 4 on the right hand side (ex: 0/4 1/4 etc). Is the spawning algorithm thread being cut short or something?