Before condemning the riser board, try hosing down the slot on the motherboard, the edge connector on the riser, and all of the PCI slots on the riser in Deoxit Gold G5. If that's not available, CRC 2-26. Who knows how those parts were stored, they probably have oxidation on them causing connection issues.
The freezing is likely caused by the same issue with the RAM slots, or you have bad memory. The memory stick edge connectors and the slots should be hosed down as well.
Both Deoxit Gold G5 and CRC 2-26 are safe to leave on the board while its powered up, just wipe the excess off.