Hi jhseltzer,
if another CPU does not help, the troubles are caused by a too weak CPU cooler.
With 220W, there is probably no chance to air cool the CPU. However, even water cooling could be tricky.
You should check the cooling system datasheet to be sure that the system is able to handle 220W. Additionally, you must ensure an appropriate air cooling of the motherboard, in particular chipset and RAM. One of the troubles with water cooling is that the huge airflow generated by the CPU is missing causing an inadequate cooling of the motherboard.
Thinking about your problem, I could imagine that the huge load caused by boinc is maybe coming too fast causing that the water cooling system is not able to react and to reach its nominal operation quickly enough. It would be interesting if you could succeed to increase the CPU load more progressively, e.g. within 5 or 10 minutes, giving time to the water cooling system to reach a stable operation. Is it possible to monitor the water temperature?
I suspect that the water temperature is going very hot between the 5 to 100% CPU load and the time the cooling system is becoming efficient (i.e. being able to evacuate the produced energy). You have to consider that there are several "interfaces" in the physical configuration: die to CPU case, CPU case to CPU cooler, CPU cooler to liquid, liquid to air cooler, air cooler to air, including the delay to establish an appropriate airflow by the air cooler as well as the appropriate water flow within the system. If the CPU load increase suddenly to its maximum, I doubt that the cooling system is able to maintain the temperature accurately. There will be an temporary overheating for a couple of minutes and ... that's it. If I would have to solve such a problem on industrial level, I would over size the cooling system with a factor 5 to 10. For example in chemical manufacturing, if you have to control a fast chemical exothermic process, you have to strongly over size the cooling capacity, whatever the real energy to evacuate in steady state would be.
All these problems were for me the main reasons not to switch to such CPU, even if I am an AMD supporter.
Cheers,
Yves
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PS (edited): Thinking about the possible causes for the water cooling system to fail to react fast enough, you should maybe try with a very good CPU air cooler (requiring that the computer case is big enough AND well air cooled); you could take a look on Noctua products (It is a recommendation not a purchase advice).

I sold the FX-9590 and I'm using an Athlon Phenom II x6 T1055. I have it overclocked to 3.15 GHz. I'm running all 6 cores at 85% load. I'm able to control the temp at 110 degrees F. The Athlon is better suited for heavy loads. Thank you for your response. Please understand I'm not critical of the FX-9590, but one must understand it needs a lot of cooling when pulling a continuous load. I didn't understand that issue or I wouldn't have bought it for my application. I wish AMD would build a 4.7 GHz Athlon.

You're very welcome !
Yves