* Bank unveils new probe by DoJ, SEC into relationships
* Barclays also under investigation on past U.S. power prices
* Pretax profit down 23 pct to 1 bln stg, hit by PPI charge
LONDON, Oct 31 (Reuters) - Beleaguered UK bank Barclays , already rocked by an interest rate rigging scandal, unveiled two new U.S. regulatory investigations into its financial probity and said third quarter profits fell by a fifth due to charges for the mis-selling of insurance.
Barclays said the U.S. Department of Justice and U.S. Securities and Exchange Commission were investigating whether its relationships with third parties who help it win or retain business are compliant with the U.S. Foreign Corrupt Practices Act.
The bank is already under investigation by Britain's financial regulator and fraud prosecutor into payments to Qatar investors after the bank raised billions of pounds from them five years ago to save it from taking a taxpayer bailout.
Barclays said it is also being investigated by the U.S. Federal Energy Regulatory Commission about the manipulation of power prices in the western United States from late 2006 until 2008.
FERC could notify the bank of proposed penalties as early as Wednesday, it said. Barclays said it will ``vigorously'' defend this matter.
New Barclays Chief Executive Antony Jenkins, who took over at the end of July when his predecessor Bob Diamond quit after the bank admitted rigging Libor interest rates, is in the midst of a review aimed at overhauling culture and boosting profitability, which is expected to cut jobs and the size of investment banking.
``While we have much to do to restore trust among stakeholders, our universal banking franchise remains strong and well positioned,'' Jenkins said.
The bank said its adjusted pretax profit in the three months to the end of September was 1.73 billion pounds ($2.8 billion), in line with analysts' forecasts and up from 1.34 billion a year ago, thanks to strong profits from investment banking.
But including a 700 million pound charge for mis-selling payment protection insurance would have left profits down 23 percent at 1.03 billion.
Including a 1.1 billion pound loss on the value of its own debt would have dragged Barclays to a statutory loss of 47 million pounds for the quarter.
The bank said performance during October had been affected by the ``challenging economic environment and subdued market volumes''.