To be honest, from what I am discovering about the AM3 CPU's I would be looking at that as the probable cause. Because the Memory Controller(s), there are actually 2 Memory Controllers on the CPU, are now on-board the CPU it is very rare for these kind of problems to be due to the motherboard.
When these problems occur, the last thing that most of us consider is that the CPU can be at fault, it has to be the Motherboard, PSU or something else but I would be trying another CPU (if you have one that you can use) to eliminate that as a cause of the problem. If you can then run with all 4 slots filled you know for certain that the CPU is the cause of your trouble. If it still doesn't work then it is probably the Mobo.
With these issues it is a process of elimination but, as I said earlier. you have already tried a different Mobo and RAM, to my mind that leaves the CPU and your PSU seems more than adequate to power your rig.