* Yahoo's board approves $1.1 bln Tumblr acquisition
* Plains Exploration stock up as Freeport sweetens deal
* Actavis to acquire Warner Chilcott in stock-for-stock deal
* Dow up 0.1 pct, S&P 500 up 0.2 pct, Nasdaq up 0.04 pct
By Leah Schnurr
NEW YORK, May 20 (Reuters) - U.S. stocks rose slightly on Monday as acquisition activity gave support, but investors were hesitant to rush into the market with indexes hovering around record levels.
Deals, including Yahoo's $1.1 billion bid for Tumblr, indicate that companies continue to search for growth through acquisitions, a bullish sign for stocks. Yahoo was up 1.2 percent at $26.83.
The Dow industrials and the S&P 500 finished Friday at fresh record highs and the Nasdaq Composite is at its highest since late 2000. A light economic and earnings calendar could leave the market vulnerable for a pullback, but those have been shallow and short-lived as investors take any weakness as a new chance to increase long positions.
On Monday, the Dow climbed to an all-time intraday high at 15,391.84, while the S&P 500 edged up to a new intraday record high at 1,672.84. Both major indexes are up about 17 percent for the year so far.
Even so, investors have remained wary of becoming too enthusiastic, which has helped keep the rally from getting overdone, said Mark Lehmann, president of JMP Securities, an investment bank based in San Francisco.
"The more days we go up and the more people who are in disbelief that we go up every day, the better," Lehmann said. "This is the least well-regarded bull market I think we've seen in a very, very long time."
The Dow Jones industrial average edged up 14.44 points, or 0.09 percent, to 15,368.84. The Standard & Poor's 500 Index gained 2.43 points, or 0.15 percent, to 1,669.89. The Nasdaq Composite Index added 1.33 points, or 0.04 percent, to 3,500.40.
Actavis rose 2.4 percent to $128.53 after the company said it will acquire Warner Chilcott Plc in a stock-for-stock transaction valued at $5 billion. Warner Chilcott gained 2.9 percent to $19.77.
Freeport-McMoRan Copper & Gold and Plains Exploration & Production said they would pay shareholders more in dividends if they approved Freeport's roughly $6 billion takeover offer for Plains. Plains shares jumped 7.3 percent to $48.85, while Freeport gained 0.7 percent to $32.90, reversing an earlier decline.
Websense Inc shares surged 28.6 percent to $24.73 after the company agreed to be acquired by Vista Equity Partners.
Shares of Tableau Software and Marketo continued to climb after both companies' shares began trading last Friday. Tableau gained 16.7 percent to $59.23, while Marketo jumped 12.3 percent to $25.95.
"That tells you investors are looking at new ideas, and dusting off some old ideas," Lehmann said. "People are looking for growth."
Federal Reserve Chairman Ben Bernanke's testimony before Congress on Wednesday is seen by many as the highlight of the week for markets. The beginning of the end of the Fed's massive bond-buying program, which has given strong support to stock gains, might come sooner than many investors think if recent gains in the U.S. labor market hold.
The Fed will also release minutes from its most recent policy-setting meeting on Wednesday, which will be parsed for signs of the direction of monetary stimulus.