International Business Machines Corp. yesterday announced that it is considering a plan to sell about one-third of its 20 percent stake in Intel Corp., a leading Silicon Valley semiconductor company, to raise $300 million.

At the same time, the world's largest computer company said it had entered into a volume purchase agreement with Intel that requires IBM to buy an undisclosed "substantial portion" of its semiconductor products from the Santa Clara, Calif.-based company through 1990. IBM is Intel's largest single customer.

Spokeswomen for the two companies said the two announcements were unrelated.

Pending acceptable market conditions, IBM said it was considering a $300 million debt offering convertible into Intel stock because "this would enable IBM to raise money at an interest rate significantly below what we would have to pay in a straight debt offering," according to an IBM spokeswoman. The conversion price would be offered at a premium to the current market price of Intel stock, IBM said.

Intel yesterday closed at $27.75, down $1.50 as the second most actively traded over-the-counter stock with almost 3.9 million shares.

"We are really quite neutral on the announcement from IBM," said Charlene King, an Intel spokeswoman.

In 1983, IBM caused a stir in Silicon Valley when it purchased a 12.5 percent stake in Intel for $250 million. The purchase represented IBM's first major equity stake in a key supplier. Since then, IBM had increased its Intel stake to 22.5 million shares -- roughly 20 percent of the company.

At the same time, Intel has become an increasingly important supplier of microprocessors for IBM's line of personal computers, as well as other semiconductor devices for IBM systems. IBM accounts for roughly 15 percent of Intel's business.

Spokeswomen for both companies declined to say how much new business the IBM purchase agreements would generate for Intel.

Intel, which has been racked with layoffs over the last year, posted 1985 revenue of $1.5 billion. Net income was $2 million with earnings barely a penny a share.

IBM said that the offering was purely for financial reasons and was not a reflection on Intel's recent financial performance.

"This doesn't indicate in any way that IBM is dumping Intel," said Richard L. Whittington, a semiconductor analyst with Prudential Bache.