a problem with Model Master paints

Miller41's question about thinning Model Master enamels prompted me to ask if any of you have had problems with MM enamels turning into a thick, useless gel? I have tossed several (7-8) bottles, some almost full, over the past few years that had become VERY thick and gel-like. NO kind of thinner could "bring 'em back" to useful life. My LHS guy said it sounded like a contamination problem. I had been using wooden toothpicks or bamboo skewers to stir the paint so maybe it was a reaction to those. My wife works in a dental office (lot's of good modeling supplies there!) and she now supplies me with plastic mixing sticks. And, it does seem to have helped. I like the MM enamel but not as well as Humbrol. I have never had a similar problem with Humbrol enamels. BTW, I have Testor's enamels in the 1/4 ounce bottles that were bought circa '65 or '66 that are STILL viable paint. Hehe, the tops of the lids say ".19 cents". Pretty good investment, eh what?