I would put money on it being your CSC. Even if your fluid level isn't dropping, the seals on the CSC could still be compromised and letting air get into the system. It's a common problem.
Don't get another OEM CSC if you can avoid it. There are several aftermarket ones available (check Z1 and ZSpeed). Make sure you also replace the CMC at the same time.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |