Feb. 28, 2012  -- American pilots at an Alaskan military base have reported a sudden spike of incidents in which they experienced an apparent lack of oxygen while flying the nation's most sophisticated fighter jets -- a mysterious, recurring problem that already caused the $77.4 billion fighter jet fleet to sit idle on the tarmac for months last year.

In at least three incidents in the last two weeks, pilots of the $143 million-a-pop stealth F-22 Raptors at Joint Base Elmendorf-Richardson reported the "hypoxia-like" symptoms, leading the base to ground their F-22s for a day for "review," Air Force spokesperson Lt. Col. Regina Winchester told ABC News.

"In each case, appropriate procedures were applied," Winchester said, and the planes went back in the air the day after the temporary halt. An additional case of a pilot experiencing hypoxia-like symptoms also popped up at Virginia's Joint Base Langley-Eustis earlier this month, another Air Force spokesperson, Lt. Col. Edward Sholtis, said.

Joint Base Elmendorf-Richardson is home to the F-22's only fatal crash -- one brought on at least partially by an unknown malfunction that caused the plane to automatically cut off the pilot's oxygen supply during a training mission.

The Air Force has been struggling since 2008 to determine why its pilots have suffered relatively rare but repeated "physiological events" involving hypoxia-like symptoms while flying the F-22s -- about two dozen of them out of thousands of training missions flown. 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.

Despite its rarity, the breathing problem became such a concern that in 2011 the entire fleet of planes -- around 180 jets that cost taxpayers some $77.4 billion total -- was grounded for nearly five months while the Air Force investigated the F-22s' life support systems. The Air Force never found the cause and cautiously sent the planes back in the air in September 2011.

The problem, however, persists.

According to Air Force numbers provided to ABC News, pilots have reported nine unexplained instances of suffering "hypoxia-like" symptoms during flight since the grounding was lifted -- compared to a total of 12 announced by the Air Force in the more than two years prior to the grounding. Sholtis said that new monitoring systems and greater pilot awareness of potential hypoxia-like effects could account for the relative uptick in cases.

Air Force: Deadly Crash Did Not Lead to Grounding

The Air Force said the nationwide May 2011 grounding was unrelated to the November 2010 F-22 crash that claimed the life of fighter ace Capt. Jeff Haney. Haney's plane went down in the Alaskan wilderness seconds after a mysterious malfunction caused the plane to automatically cut off his oxygen system.

After an investigation into that crash, the Air Force blamed Haney, saying he was apparently too distracted by not being able to breathe to properly fly the plane. Hypoxia did not play a role in the crash, the Air Force report said.

READ: F-22 Fighter Crash Questions Linger, Pentagon Reviews Air Force Report

Along with the F-35 fighter, which is less slightly expensive per plane, the F-22 marks America's foray fifth-generation stealth fighter jets that the Air Force said can dominate the air space anywhere in the world -- even if they've never had to prove it. Not a single one of the Raptors has been used in combat operations from Iraq and Afghanistan to Libya since they went combat ready in late 2005.

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

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