NEW YORK (CNNMoney.com) -- Stocks struggled midday Monday after North Korea nuclear tests and higher oil prices unnerved investors, despite several key corporate deals in the works.

The Dow (down 20.90 to 11,829.31, Charts) lost 0.1 percent two hours into the session. Last Thursday, the Dow hit a record close, ending at 11,866.69 and briefly touched a trading high of 11,870.06.

The broader S&P 500 (down 1.12 to 1,348.46, Charts) index hovered just below breakeven and the tech-fueled Nasdaq composite (up 0.52 to 2,300.51, Charts) added 0.2 percent.

North Korea claimed it conducted a successful underground nuclear test Monday, according to the country's official Korean Central News Agency. The U.S. government said it was looking into the tests but believed the claims to be true. (Full story.)

But considering the nuclear tests in North Korea and oil prices up nearly a dollar, the market is doing extremely well, according to Art Hogan, chief market strategist at Jefferies & Co.

Cablevision Systems (up $2.70 to $26.63, Charts) jumped 11 percent after the Dolan family said it offered to buy the remaining shares it does not already own in a deal that values the company at $7.9 billion.

PNC Financial Services Group (down $4.02 to $69.58, Charts) said it agreed to buy Mercantile Bankshares (up $7.72 to $44.50, Charts) for $6 billion in cash and stock, sending shares of the regional bank down more than 6 percent.

And shares of GlaxoSmithKline (down $0.16 to $54.15, Charts) edged lower after the drugmaker said it's agreed to buy CNS (Charts) for $566 million, or $37.50 a share.

In broker action, Kroger (down $0.15 to $22.34, Charts) was downgraded to sell from neutral at Banc of America Securities, sending shares of the grocery-store chain down slightly.

Mastercard (down $3.63 to $71.52, Charts) sank 5 percent after Goldman Sachs and Citicorp both downgraded the credit card company.

Sprint Nextel (down $0.43 to $18.05, Charts) also fell nearly 2 percent after the wireless provider was downgraded by UBS to neutral from buy and Verizon (down $0.26 to $36.54, Charts) fell nearly 1 percent after a Citigroup downgrade to sell from hold.

On the upside, Citigroup upgraded AT&T (up $0.23 to $32.01, Charts) to buy from hold and Advanced Micro Devices (up $0.05 to $24.06, Charts) was upgraded to neutral from reduce at UBS.

Market breadth was negative. On the New York Stock Exchange, losers edged out winners on volume of 450 million shares. On the Nasdaq, decliners beat advancers four to three on volume of 550 million shares.

U.S. light crude oil for November delivery gained 80 cents to $60.56 a barrel on the New York Mercantile Exchange.

COMEX gold for December delivery rallied $4.50 to $581.30 an ounce.

Stocks in Asia fell, with South Korea's main stock index tumbling 2.4 percent and Hong Kong's Hang Seng index losing more than 1 percent. Japan's Nikkei stock index was closed for a holiday.

European shares were mixed in late trade.

The dollar reached eight-month highs against Japan's yen.

Treasurys were not trading due to the Columbus Day holiday.

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