The news raced through Silicon Valley like a burst of electrons. Steven Jobs, 33, co-founder of Apple Computer and one of the world’s most famous entrepreneurs, was set to unveil the machine he had been laboring on since he stormed out of Apple nearly three years ago. The computer press, having first trumpeted the device’s imminent debut last October, then again in February, then March, then May, was crackling anew with anticipation. This time it was certain. On June 15, or at the latest June 16, the world would finally see the computer that Jobs has billed as the technological successor to the Apple II, the IBM PC and the Macintosh — a machine known only by the name of his new company, NeXT. But as the due date approached, rumors of development problems began to leak out of NeXT’s Palo Alto headquarters. Last week company sources confirmed what many already suspected: Jobs would miss yet another deadline, by several weeks at the very least. The setback did nothing to allay the intense speculation surrounding the machine, but it did raise troubling questions about the computer’s ultimate chances for success and about the formidable reputation of its creator. By all accounts, Jobs’ new machine is an engineering marvel. People who have seen prototypes describe a sleek, black magnesium cube with a space underneath where a keyboard can be neatly hidden away, a stereo sound system that rivals the crisp tones of a compact-disc player, and a jumbo 17-inch black-and-white display screen capable of visual pyrotechnics that are often characterized as “drop dead.” The machine’s software is reported to be controlled by a powerful, versatile operating system called Mach, a variation on AT&T’s popular Unix system.