Inspiron 660s Memory SLOT Failure?

Here's a VERY strange problem: Inspiron 660s w/ two 4G DDR3 DIMMs installed  was up and running when it suddenly turned off with flashing amber power light and two-beep code indicating memory problem. When power cycled,the same symptom occurred (flashing amber & two-beeps).

Troubleshooting showed DIMM1 slot was bad (Item 9 on p.15 of User's Manual).If either 4G DIMM card was installed in the DIMM2 slot (Item 11 on p.15) the system would boot OK.

Both slots populated: Flash/beep.

DIMM1 slot populated with either 4G DIMM: Flash/beep.

DIMM2 slot populated with either 4G DIMM: Boots OK.

After "fixing" the system by de-populating the DIMM1 slot, full Dell Diagnostics (online version) were run with all PASS results. Memtest86+ passed as well.

QUESTIONS:

How could a memory *SLOT* (not memory card) suddenly fail in the middle of operation? The system was up & running but nothing was being done on it at the time of failure. Did a part of the chipset fail?

Is there any way to fix this problem short of replacing the MoBo?

Thanks.