1994 GMC Sierra Why does transmission shift sometimes and n

Transmission will sometimes shift out of second into third and other times not. I had the computer scanned and there were no codes to read. Everything is A okay. I know it is a electrical problem but where does one start to look. What should the volt reading be on the VSS. The reading I had was 0.5 volts. And some one has also suggested the ignition switch on column, (not the lock cylinder). Complex by problem. Any suggestions.