I saw those from Cablesaurus ... not sure with the molex connector is for. But this could explain a lot due to differences in power specifications:

"All sizes of ×4 and ×8 PCI Express cards are allowed a maximum power consumption of 25 W. All ×1 cards are initially 10 W; full-height cards may configure themselves as 'high-power' to reach 25 W, while half-height ×1 cards are fixed at 10 W. All sizes of ×16 cards are initially 25 W; like ×1 cards, half-height cards are limited to this number while full-height cards may increase their power after configuration. They can use up to 75 W (3.3 V/3 A + 12 V/5.5 A), though the specification demands that the higher-power configuration be used for graphics cards only, while cards of other purposes are to remain at 25 W.[10][11] Optional connectors add 75 W (6-pin) and/or 150 W (8-pin) power for up to 525 W total (75 W + 3×150 W)" from http://en.wikipedia.org/wiki/PCI_Express#Power

So by plugging a x16 card into a x1 riser, you're only able to pull 10-25W max from the PCI-E side. A graphics card might be expecting 25-75W, hence the instability.