There could be several things at play here.Your vet has cerainly done very well to help rule out a lot of potential causes such as hypokalemia, hypercalcemia and hypoglycemia.
One cause of 'drunk walking' or ataxia in cats can be discospondylitis or infection of the intervertebral disk. If this occurs high in the spinal cord (e.g the neck area) it can cause signs in all 4 limbs. The best way to diagnose discospondylitis would be a myelogram (an xray of the spine taken after contrast has been injected into the spinal canal to outline it on films), sample of cerebrospinal fluid is usually collected at the same time (CSF tap) and cultured (to see if bacteria will grow) as well as analysed for protein levels and cell counts that may indicate inflammation.
As your girl is older there is also a possibility of a tumour in the brain causing these signs, also a tumour of the membranes covering the brain that may grow and press on it can cause ataxia in a cat. The only way to diagnose tumours within the skull is an MRI scan.
I would have expected prednisolone (that she is on for asthma) to help if there is an immune-mediated component to this (such as immune-mediated polyarthritis/vasculitis), however, if infection is present a very long course of antibiotics would be necessary and steroids alone are not enough.
If you are certain that your girl could not have ingested any toxins, or be suffering from a electrolyte imbalance (a simple blood test would rule this out) then the only way to diagnose the condition further is specific tests concentrating on the central nervous system. Such as a myelogram, CSF tap, or MRI and CT scans.
Even though what you are describing does not sound like a classical seizure there also is a chance that your kitty is exhibiting signs of a petit mal / partial seizure. The diagnosis of epilepsy is usually reached after everything else that could cause the signs is ruled out.
Seizures can ge classed into 3 different groups:
Seizures resulting from metabolic problems or toxicity (i.e., when the brain itself is normal) are called REACTIVE SEIZURES.
Seizures resulting from an identifiable brain abnormality are called SECONDARY SEIZURES.
Seizures for which neither of the above problems apply (i.e., when no cause can be found) are called PRIMARY SEIZURES.
Again a blood panel is the first step in diagnosis (and would be worth repeating if the last one was performed a year ago) followed by more specific tests.
If you are concerned about the 'drunk' state of your cat or if she does not seem to come through this episode by herself as she did last time I recommend that you take her to your local Vet or ER Vet for examination.
I hope this helps, it is a difficult condition to attempt to diagnose without a hands-on exam. Please let me know if I can be of more help.