Maryland Cup Corp., maker of Sweetheart cups, is flirting with merger again.

The Owings Mills company, which is the nation's largest manufacturer of disposable food containers and utensils, announced yesterday that it is entertaining an unnamed suitor in "preliminary merger discussions."

The announcement marked the third time since 1979 that Maryland Cup has had such talks. Discussions with Kraft Inc. in 1979 and with Fort Howard Paper Co. in 1981 failed to produce merger agreements.

There was some speculation on Wall Street yesterday that Fort Howard once again was trying to woo Maryland Cup, but neither company would comment on the gossip.

"We simply do not give any information out--whether we are, or whether we are not in the process of negotiations," said a spokeswoman at Fort Howard, which had $471 million in sales last year.

"We have issued a statement," said a spokesman for Maryland Cup, acknowledging that the statement contained little information about the negotiations under way.

But even the gossip was good news on Wall Street yesterday. Maryland Cup's stock shot up 5 3/8, closing at 44 3/8, marking the best performance yesterday on the New York Stock Exchange and strengthening Maryland Cup's position in any merger talks, according to two analysts.

"The company is a magnificent property, a very successful firm. It is a market leader in virtually every field where it competes," Clarence W. Brown of Lehman Brothers said yesterday. Asked why he thought the company was seeking merger, Brown said: "I guess it comes down to the point of everything having a price."

That price, should it be negotiated, probably would be dear, said Jeffrey F. Morris of New York-based Evans & Co. Maryland Cup has said in previous negotiations that "it would entertain bids substantially above market price," Morris said.

Maryland Cup was founded in 1910 by four Russian immigrant brothers, Joseph, Isaac, Nathan and Samuel Shapiro.

Maryland Cup had $579 million in sales and 10,700 employes in 1982.