two DIMM Slots are not working

You can make an appointment at the Apple Retail Store - Genius Bar and take it in for service. You can also look for an Apple Store - Find a Store - Apple.

Exchanging the cards showed that all memory cards are working fine.

...working fine at the time that you checked them.

It would be really nice if once a DIMM made an error, it would fall over dead. But that is not how they work, and it is made much more complex by the Error-Correction Memory in these Macs.

The error-correction logic is applied very aggressively at Startup, and any DIMMs that experience any error (correctible or not) during those few seconds will have its slot declared "empty" and that DIMM will not be used. But there is no historical carry-forward -- the next time you do a restart, they are all tested from scratch, and DIMMs that failed last time may pass the next time.

When a correctible error occurs in the running system, it is corrected in Hardware in one extended memory cycle, and the errors bits are checked and tabulated by a background process. They are available as a STATIC read-out using this display.

About this Mac > ( System Report ) > Memory:

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Radio Shack used to sell contact cleaner is a spray can with a stiff brush tip. I am not sure where you would get that today.

http://www.amazon.com/WD-40-Specialist-Electrical-Contact-Cleaner/dp/B00CMT9TFY

http://www.homedepot.com/p/CRC-QD-11-oz-Contact-Cleaner-02130-6/202262505

http://www.walmart.com/ip/CRC-Electronic-Cleaner-11-Oz./16817418

I do not think that the slots are really replaceable since they ae soldered to the logic board.

To replace the slots requires special equipment.

Apple would do a repair-by replacement on the motherboard. You could ask for that if under warranty, otherwise it may be many hundreds of dollars.

The impact of leaving a slot open should be less than five percent of memory speed, but you cannot get to "all slots filled" total memory.