Alessandro Matri's strike 12 minutes from time sank second-placed Juve, giving leaders Inter Milan the chance to extend their advantage over their old rivals to nine points with a home win over Torino on Sunday.
Claudio Ranieri's side, who lost 2-1 at Udinese on Wednesday, went behind on 16 minutes when Davide Biondini outjumped Marco Marchionni to head in at the far post.
Cagliari held Inter to a draw earlier this month and the Sardinians went one better when striker Jeda finished off a quick breakaway on 54 minutes before Matri scored a carbon-copy winner with Juve's defence nowhere.
Stuttering Napoli were 2-0 up after 27 minutes thanks to Ezequiel Lavezzi's deflected shot and a header from midfielder Marek Hamsik, back following a two-game ban.
Like all Italian games this weekend, the two matches kicked off 15 minutes late because of a protest over the Court of Arbitration for Sport banning Napoli's Daniele Mannini and Brescia's Davide Possanzini for a year for being late for a 2007 drugs test.
