* New home sales data due
* Pandora falls in premarket after outlook
* Futures: Dow flat, S&P up 1 pt, Nasdaq up 3.5 pts
By Chuck Mikolajczak
NEW YORK, Aug 23 (Reuters) - U.S. stocks were poised for a flat on Friday, a day after the Nasdaq stock exchange suffered a historic trading halt of roughly three hours as a result of technical problems.
Trading in thousands of U.S. stocks ground to a halt for much of Thursday after an unexplained technological problem shut down trading in Nasdaq securities, the latest prominent disruption to the operations of U.S. markets.
Chief Executive Officer Robert Greifeld said on CNBC television the Nasdaq resolved the technical issues that led to Thursday's trading halt, but cannot guarantee there would be no future problems.
Even with the Nasdaq outage on Thursday, the S&P 500 managed to register its biggest percentage gain since Aug. 1, but was unable to close above its 50-day moving average for a fifth straight session. The mark, now at 1,658.87, has become a technical hurdle.
The benchmark S&P index is on pace for a slight advance for the week after two straight weekly losses, as investors exercise caution amid uncertainty over how soon the Fed will begin to wind down its $85 billion a month stimulus program.
"This has been a very unique market situation with the Fed stimulus being such an important component to the market rally. This is uncharted waters for us," said Gordon Charlop, managing director at Rosenblatt Securities in New York.
"So regardless of what the move is, the fact you are someplace you haven't been before is cause for uncertainty."
The next Fed monetary policy meeting is scheduled for Sept. 17-18.
U.S. economic data expected on Friday includes new home sales for July at 10 a.m. (1400 GMT). Economists in a Reuters survey forecast a total of 490,000 annualized units, compared with 497,000 in June.
S&P 500 futures added 1 point and were roughly even with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were flat and Nasdaq 100 futures rose 3.5 points.
Pandora Media Inc dropped 6.6 percent to $20.27 in premarket trade after the Internet radio service said late Thursday that rising expenditures to acquire music and expand its sales force would push fiscal 2014 earnings below analyst expectations.
Aeropostale Inc tumbled 15.5 percent to $9.28 before the opening bell after the teen apparel retailer forecast a deep third quarter loss.
Expedia Inc rose 5.7 percent to $49.10 in premarket after the online travel company announced a strategic marketing agreement with Travelocity.