Sources close to Cassidy's family tell TMZ ... David's ashes have been tucked away in a cremation unit since his November. David's ex, Maura Rossi, is now nuclear, claiming something untoward is going down.

Rossi thinks David has already been forgotten by his entire family and thinks they are dishonoring his memory by letting his ashes sit in storage. She says she's so upset, she can't even sleep or eat.

But here's the deal. We're told the plan all along was to wait until this summer to spread David's ashes in a place that was near and dear to his heart ... Saratoga Race Course in Saratoga Springs.

David was a horse racing nut, and wanted his ashes spread around the 154-year-old race track, which he frequented for more than 3 decades.

It's unclear when exactly the family's fulfilling David's final wish ... but, it's a good bet it'll come on August 18, when Saratoga hosts the "David Cassidy Memorial Race." Benches at Saratoga and the National Museum of Racing and Hall of Fame already bare his name.

As we reported ... David died after his organs failed. He was 67.