The ruble lost one-fourth of its value against the greenback Tuesday in the worst one-day nosedive since 1992. It closed at 3,926 to the dollar, or 1/40th of a cent, at the Moscow Interbank Currency Exchange, a drop of 845 points. Russians went on a buying spree, picking up durables like refrigerators, stereo and televisions -- anything to rid themselves of their national currency. What caused the drop? Speculators dumping the currency in huge quantities, and an underlaying pessimism about the country's political stability.