This has just started to appear for me too, only witnessed it in Mor Dhona but also just had an incident in coerthas when spawning a quest enemy it froze for a few seconds and the enemy spawned invisible only its weapon was visable.

Only after decent periods of time too. Been playing since release and not noticed this error until the last few days.

i7 920 @ 3.36 (160 x 21)
12 GB Ram
HD 7950 14.4 catalyst (previously HD7870 which broke about a month ago never had issue, also used a HD 4970 as a stopgap which I think perhaps suffered a similar issue but was more related to overheating which was fixed by me turning the fan up)

Win 8.1 64 bit