According to a post over at Hardware.fr and other industry sources, PLX and its PEX8747 PCI-Express Gen 3.0 bridge may be the reason for delays seen in GPU and motherboard markets.
PEX8747 is currently the only PCI-Express Gen 3.0 bridge chip on the market. It has been used on most high-end motherboards, Nvidia's dual-GPU GTX 690 graphics card and most probably the recently pictured dual-GPU HD 7970 X2 graphics card as well.
According to available info, PLX has experienced a high demand for the chip. While some companies managed to get the inital batch of the chip, others were not that lucky.
The same reason could be behind the HD 7990 delay. As you already know, there has been a lot of talk about that card and although we've heard rumors it could show up at Computex, they quickly went missing.
Of course, AMD could use this delay to pair up some Tahiti XT2 chips for the company's HD 7990 cards, bringing higher clocks and lower power consumption to the table. Nvidia, on the other hand, is stuck with the launched GTX 690 which is really hard to find.
Of course, PLX is probably ramping up production and things should be better eventually.
Via Techpowerup.com.
PLX bridge chip in shortage