Hi I Have Vertigo Like Symptoms When I Stand, But

hi
i have vertigo like symptoms when i stand, but it always turns into overall body weakness with muscle twitches, falling, and on occasion, complete syncope. these symptoms usually last from 30 seconds to 2 to 3 minutes in duration. the more intense the muscle twitching, the more likely i will pass out.
i have had this problem ever since i had chemotherapy five years ago for cancer. my oncologist claims there i a slight chance this problem may be permanent, however, he claims side effects may take years to disappear.
i have seen most specialists such as neurologists, cardiologists, ent physicians. none can explain what is happening as far as their specialties are concerned.
i have had extensive therapy for vertigo without any improvement in symptoms and after five years everyone involved in my care are clueless
i have had these symptoms ever since i had chemotherapy 5 years ago.