The actor and radio host tells TMZ ... after seeing hundreds of doctors for a mystery illness, in March he was finally diagnosed with hydrocephalus ... a neurological disorder caused by a buildup of liquid in the brain.

Surgery is set for Monday, and the hope is it can alleviate the symptoms.

Last April, "The Partridge Family" star began battling the mystery illness after his wife noticed his speech was off and he lost the ability to walk or balance. He was hospitalized for 5 days and underwent a battery of tests. Doctors ruled out a stroke, but couldn't come up with a diagnosis.

While Danny says no one knows for sure how he got in such bad shape, he wonders if it's related to all the wear and tear he's put his head through over the years ... like taking guitars to the face and boxing Jose Canseco.

Danny says the surgery will drain the liquid from his brain through a shunt.

In the meantime, Danny's been doing his radio show from home.

Of course, Danny acknowledges there's always a chance the surgery won't fix everything ... but it sounds like he's going into the operating room with the right frame of mind.