Xerox, founded in 1906 as a photographic paper manufacturer, is still doing fairly well today. It had a brief flirtation with computing in the 1970s, through its PARC subsidiary, but quickly brushed it off and stuck to the knitting. So that's 114 years and still doing well - unusual for any corporation, especially in a technical industry.
HP, founded in 1939, was a name synonymous with engineering excellence (and high levels of integrity) for about 50 years. Then everything gradually began to go wrong. HP hardware has always been pretty good and reliable - from servers through PCs and workstations to printers, storage and calculators - but somehow the company's top management always found ways to screw everything up. Buying Compaq/DEC/Tandem might have been a brilliant move, but turned out to be an extremely indigestible meal.
Maybe it's as simple as this: in 1993 Bill Hewlett and David Packard left the board of directors. Just as DEC plunged to its destruction when Ken Olsen was persuaded to leave. Those three had amazing values and ethics, which somehow spread throughout their companies. They were succeeded by standard 1990s money-chasing managers, who had no vision and no moral aspirations.