It seems a lot of folks jumped on the idea that the leak is from the neck when the OP only stated that he "thinks" it is from the neck.
If the leak is coming from the neck due to a size issue, then I am a bit fuzzy on how one can do 6 dives and flood-out each time and not have a clear idea that they are taking in water from that area.
As others have mentioned, neck hair, adams apple, and neck tendons when turning or lifting the head can all cause issues with a neckseal, even more so if it is too large.
The fact that the neck seal was replaced should have ruled out an improperly glued or detaching neck seal and any small tears...unless the glue job was done poorly and/or the replacement seal had a defect.
My thinking is, that since the OP is not
sure that water is leaking in through the neck that the problem is more likely to be with the zipper. I would also check that the inflation and dump valves are adequately tight.
The Aqualung Fusion One is a back-entry drysuit made from waterproof/non-breathable material. It comes with latex seals at the wrists and neck. Aqua Lung does not market this suit with the Si-Tech silicone neck and wrist seal systems, though one can of course add them as an aftermarket item.
Because the drycore material of the Fusion One is non-breathable, could it be that some (or all) of the wetness is due to absorbed sweat?
Since the Fusion One is a back inflate, the OP may not be ensuring their zipper is fully closed or the zipper is damaged.
Perhaps the OP can provide some insight as to how much water is filling the suit and what part of their body/undergarments are getting soaked.
If I were the OP I would obtain and suspend the suit by ball shaped mooring float/boat bumper to effectively block the neck. then remove the skin from the suit, and use some water bottles or empty cans to plug the wrist seals. Inflate the suit and spray suspect areas with mixture of water/dishsoap/glycerin...the glycerin makes the bubbles a bit stronger thus making them more likely to be visible.
A mooring float should only cost about $10....attach some line (cord/rope) to it and suspend the suit:
Here is a picture of the core of my suit with the skin removed suspended and inflated with a similar float inserted...instead of cans/water bottles I have my drygloves attached to block the wrists. The chair was put there to provide perspective for the photo and for me to stand my tank on so I could connect my low-pressure inflator to the suit and fill it up. Glycerin for the soap mixture can be purchased at a pharmacy.
-Z