International Business Machines Corp. said yesterday that its profit fell 12.9 percent in the second quarter from a year earlier as a result of a listless economy, a strong dollar and disappointing shipments of some computers.

Intel Corp. and Consolidated Rail Corp. also posted lower earnings for the period, while in the banking sector, Chase Manhattan and J. P. Morgan & Co. said profits were sharply higher. Continental Illinois National Bank also reported second-quarter results.

The decline at IBM, which had been expected, was the second consecutive quarterly setback for the world's largest computer company, and analysts were divided on whether the firm was out of the woods.

For the second quarter, profit fell to $1.41 billion ($2.30) from $1.62 billion ($2.65) a year earlier. Revenue rose 2.1 percent to $11.43 billion from $11.20 billion.

Early this year, IBM said it would have a difficult time showing any growth in the first half of 1985 but that it expected a strong second half. In May, IBM had said a slight decline in earnings was probable in the second quarter and in June it said it probably would not meet its earlier objective of solid growth in profit and revenue for all of 1985.

For the first half of the year, earnings fell to $2.40 billion ($3.91), from $2.83 billion ($4.62) a year earlier. Revenue rose to $21.20 billion from $20.78 billion a year earlier.

*Slumping semiconductor demand pushed Intel Corp.'s second-quarter net income down 80 percent from the same period last year, the company said. Earnings were $9 million (8 cents per share) for the second quarter ended June 29, down from $55 million (39 cents) for the same period last year.

First-half profits totaled $20 million (17 cents), compared with $105 million (90 cents) a year ago. Revenue was off 6 percent to $735 million from $782 million for the first six months of 1984.

Intel makes memory and microprocessor components and systems.

*Conrail reported net income of $154.8 million ($5.64 per share) for the second quarter and $209.5 million ($7.60) for the first six months of 1985. That compared with record net income of $178.9 million ($6.64) and $269.8 million ($9.99), respectively, for the similar 1984 periods, the nation's federally subsidized freight railroad corporation said.

Conrail noted in a news release that it did not pay industry scale wages in the first half of 1984. If it had, Conrail said, 1984 earnings would have been about $147 million for the second quarter and $205 million for the first half.

Revenue for the 1985 second quarter was $838.4 million, from $886.7 million in 1984. For the first six months, revenue was $1.641 billion, from $1.743.8 in 1984.

*Among banks, Chase Manhattan Corp. said its earnings rose 45 percent in the second quarter, while J. P. Morgan & Co. reported a 51.8 percent jump. Chase put Argentine interest payments in loan-loss reserves and Morgan did not count them at all.

Chase, whose principal subsidiary is the third-largest U.S. bank, said its $13 million share of $570 million in interest payments received from Argentina was included in interest income. But Chase put the payment in its $105 million provision for possible loan losses, up from $75 million in the second quarter of 1984.

Morgan, holding company for No. 5 Morgan Guaranty Trust, one of the most conservative of large banks, said it deferred $18.6 million in Argentine interest payments. Morgan said the payments are listed as "other liabilities" until Argentina's refinancing package has been completed.

Morgan also doubled its provision for loan losses to $90 million from $45 million last year, reflecting "management's concern about the uneven economic recovery both in the United States and abroad."

Chase earned $131 million ($2.88 a share) in the second quarter, up from $90 million ($2.21). The per-share income reflects a preferred offering issued in connection with the purchase of Lincoln First Banks and a higher number of common shares outstanding.

Morgan earned $157.4 million ($1.75) in the second quarter, from $103.7 million ($1.16) a year ago.

Continental Illinois Corp., meanwhile, reported earnings of $37.3 million (13 cents) in the second quarter and said year-earlier results were meaningless in light of last September's near failure.

John E. Swearingen, who took over as Continental chairman after a run by large depositors nearly pushed it under, said the company made "significant continued progress in reducing our dependence on Federal Reserve borrowings and special funding provided by a group of U.S. banks."

In the first quarter, the bank earned $39.3 million (14 cents), bringing six-month totals to $76.6 million (27 cents).