Jan. 13, 2012  -- Pilots for the F-22 fighter plane have reported several new instances of experiencing "hypoxia-like" symptoms while at the controls of America's most expensive and sophisticated stealth jet, the Air Force said, an apparently rare but potentially deadly oxygen problem that has stumped the military for the last four years.

From 2008 to 2011, pilots for the $143 million-a-pop stealth jet reported at least 12 incidents of experiencing the "hypoxia-like" symptoms, prompting the full fleet of F-22s to be grounded in May 2011 while the Air Force investigated. After an intense, nearly five-month investigation, the Air Force said it could not figure out what could be making the pilots feel the effects of hypoxia and cautiously sent the birds back into the skies in October.

But the Air Force told ABC News the problem persists -- in the 6,000 sorties flown since the grounding, pilots have reported another eight instances of suffering "hypoxia-like symptoms." In each of the new cases, the pilot followed proper procedures, returned to base and landed "without incident," the Air Force said.

"The Air Force has not yet identified a root cause or a single mechanical deficiency, but through a range of both engineering and physiological actions we can mitigate the risk; this includes rigorous inspections, enhanced safety procedures, new training on life support systems, improved physiological monitoring, and continued data collection," Air Force spokesperson Capt. Jennifer Ferrau said in a statement to ABC News.

READ: F-22, World's Most Expensive Stealth Fighter, to Fly Again

Hypoxia occurs when the brain does not receive enough oxygen and can cause dizziness, confusion, poor judgment and inattentiveness, according to the National Institutes of Health.

The F-22, though America's most expensive per-jet fighter, has never gone to war since going combat ready in 2005. In every major air combat operation, from Afghanistan and Iraq to Libya, the Air Force said the highly advanced fighter was not an operational necessity.

READ: The $77 Billion Fighter Jets That Have Never Gone to War

Air Force Blames Oxygen-Deprived Pilot in Deadly F-22 Crash

Two months after pilots began taking the Raptors back in the sky, in December the Air Force published its investigation into the death of Capt. Jeff Haney, a veteran F-22 aviator who died after a malfunction caused his oxygen system to fail.

The Air Force said a problem with the plane's bleed air system, a system that draws air from the engine for other vital systems including the pilot's oxygen, prompted the oxygen shut down, leaving Haney to experience "a sense similar to suffocation."

Though they acknowledged the oxygen failure, Air Force investigators said the crash was Haney's fault for being too distracted by not being able to breathe and failing to either reduce altitude and take off his oxygen mask or to activate the emergency backup oxygen system.

"By clear and convincing evidence, I find the cause of the mishap was the [pilot's] failure to recognize and initiate a timely dive recovery due to channelized attention, breakdown of visual scan and unrecognized spatial disorientation," the president of the investigation board, Brig. Gen. James Browne, said in conclusion.

READ: Air Force Blames Oxygen-Deprived Pilot in F-22 Crash

The Air Force has no plans to ground the F-22 again as they cautiously monitor their pilots in the sky and continue to look for answers, another Air Force official told ABC News.