Testor ModelMaster Dark Sea Blue paint

Could be contaminated, old (pigments and their associated solvents will degrade over time), or a processing problem at Testors with that particular lot. Save yourself the headaches, and plunk down a couple bucks for a new bottle...but don't purchase it at the same location as your current bottle. If it's a problem with particular lot, you'll be stuck again.
Gip Winecoff

You didn't say wether the paint was acrylic or enamel. If it's enamel, try adding a few drops of laquer thinner to the bottle. I've never used acrylics before so I wouldn't know what to do about a problem with that kind of paint.
Ray

No Ray, it is acrylic, which you can thin with water, alky, or Testors name thinner...none of it worked.