"The relative calm that has prevailed over credit markets since mid-March, when the Federal Reserve stepped in to help a fragile system, shows signs of dissipating," The Wall Street Journal reports. "The latest example: Moody's Investors Service Wednesday threatened again to lower the top triple-A ratings of MBIA Inc. and Ambac Financial Group Inc. because the bond insurers are having difficulty getting new business while losses mount on mortgage securities they insured. 'The functioning of financial markets has improved of late, but conditions remain strained, and some key funding and securitization markets have shown only tentative signs of recovery,' Federal Reserve Chairman Ben Bernanke said in a speech Tuesday at the International Monetary Conference in Barcelona, Spain. This is especially true in the asset-backed-debt markets that finance lending to consumers seeking credit-card debt, car loans or loans to pay for college."