I suppose that Meniere's has been ruled out? A friend of mine had that condition for a number of years but is not currently bothered by it. It was bad enough that on occasion she actually lay down on the floor. Scary and unsettling.
"A typical attack of Ménière's disease is preceded by fullness in one ear. Hearing fluctuation or changes in tinnitus may also precede an attack. A Ménière's episode generally involves severe vertigo (spinning), imbalance, nausea and vomiting. The average attack lasts two to four hours. Following a severe attack, most people find that they are exhausted and must sleep for several hours. There is a large amount of variability in the duration of symptoms. Some people experience brief "shocks", and others have constant unsteadiness. An unusual sensitivity to visual stimuli is common. (Lacour, 1997)
"A particularly disabling symptom is a sudden fall that may occur without warning. These are called "otolithic crisis of Tumarkin", from the original description of Tumarkin (1936). These are attributed to sudden mechanical deformation of the otolith organs (utricle and saccule), causing a sudden activation of vestibular reflexes. Patients suddenly feel that they are tilted or falling (although they may be straight), and bring about much of the rapid repositioning themselves. This is a very disabling symptom as it occurs without warning and can result in severe injury. Often destructive treatment (e.g. labyrinthectomy or vestibular nerve section) is the only way to manage this problem. Other otologic conditions also occasionally are associated with Tumarkin type falls (Ishiyama et al, 2003). See here for more information about drop attacks."