"Interest rates have entered a downward cycle that will likely last the next two to four weeks," says banking newsletter 100 Highest Yields. "If they are still dropping then, the pattern could stretch for months." The newsletter, which is published in North Palm Beach, Fla., tracks interest rates nationwide and has found that in recent weeks more banks are reducing interest rates on certificates of deposit than are raising them. What's more, the ratio of declines to increases is accelerating in favor of declines, usually a sure sign of more decreases to come. Also, mortgage rates have been decreasing steadily, it says. Several bankers told the newsletter that interest rates at the end of the year will be lower than they were at the end of 1988. Even though there will be ups and downs along the way, the overall trend is down, said one banker.