What @jpwhit said "A likely culprit is a small air leak somewhere on the suctions side of the raw water system. Something letting air in between where the water enters the hull fitting and the intake side of the raw water pump" is exactly right. We have the same boat and it developed an overheating problem out of the blue last fall. Turns out the sealant for the clear elbow at the inlet for the raw water strainer was faulty, admitting air into the water flow. It never leaked a drop of water, but air was getting sucked in causing the overheating. Quick and easy fix by resealing it. Also, as @ColeGiacopuzzi said, the PRV can also be the source. The Ilmor manual says they're meant to be replaced every 100 hours. Replace it with a brass one like he said. Another quick fix if that is the problem. Thermostats can go bad, rarely I'm told, so it's probably not the source of your overheating.
But there's one other possible culprit: Curiously we developed a second overheating problem 8 months after the issue with the bad sealant. This one defied diagnosis as we did all the usual checks and replacements. Nothing worked. It turned out we had a plugged cross-over tube. This is somewhat of an obscure part, and even when I had the new one to try, I couldn't find the existing one in order to replace it. It's hard to even see at the front of the motor, as it's more or less hidden under the alternator on the left side of the motor, plus everything is painted blue making it even harder to pick out. Once I found it, the replacement was relatively easy, except the alternator has to be removed and it's like a Chinese puzzle getting it completely out to expose the left end of the cross-over tube. (The right end is exposed and easy to access once you know what you're looking for). Anyhow, this tube was plugged at one end. I put the new one in and everything was back to normal. So if nothing else works, consider the cross-over tube as a last resort. You can take out the one you have, plug one end with your finger and blow through the top hose connection. Air should come out the other end. Then reverse the process to see if either side is plugged.