Credit problems creeping back up

Downgrades to the big mortgage bond insurers AMBAC and MGIC would require them to come up with more cash and put greater strain on the credit and mortgage markets.  The mortgage insurers have been in trouble, taking repeated massive write downs to their portfolios. The Wall Street Journal on the Credit Crunch Flare-Up: 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 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. Potential downgrades from Moody'