The RAM DIMMs are millions of times more likely to be the cause of the problem compared to the riser-cards.
Any tests I can do to determine the problem?
Yes, your Mac checks each and every Read from RAM Memory with a Hardware assist built into the Xeon Processor, corrects single-bit errors in Hardware on the fly, and tabulates that information in the background for you. All you need to do is your normal work, and from Time-to-Time invoke or refresh that STATIC display.
menu > About This Mac > ( More Info ) > ( Report ) > Memory
graphic from anandtech.com
And now, the leap of faith:
• modules showing Errors are BAD, and should be replaced.
• modules whose slots show as Empty, were so BAD, they threw an error (possibly a correctable error) during the first few seconds of the Power-on Self Test, and will therefore not be used by the System.
If an uncorrectable error occurs, your Mac is designed to kernel panic with a machine check, typically detected by multiple processors. Occasionally (but not often) the exact module is called out. This keeps a memory error from poisoning your data.