Aguero famously scored an injury-time winner against Rangers to seal the 2011-12 Premier League title for City, but similar celebrations appear increasingly unlikely this time around as Manuel Pellegrini's team lie eight points behind leaders Chelsea after 11 games.
Charlie Austin had time to net two disallowed strikes before his sixth goal of the season gave QPR a 21st-minute lead, with City threatening to crumble under a ferocious early onslaught.
Hart's failure to kick the ball directly out of his area meant a re-take was necessary, despite a fine finish from the former Burnley forward, but City were visibly unsettled and Hart was forced to deny Austin from close-range in the 15th minute before the same player finally got his goal.
Playing, like Austin, on the shoulder of the last man, Aguero wonderfully brought down Mangala's raking pass but the ball struck his hand as he controlled before beating Robert Green.
Isla made a vital intervention to stop Aguero converting Jesus Navas' low cross from the right channel in the first minute of the second period, before Toure came agonisingly close to converting a move he launched down the opposite flank.
City belatedly found slightly more control in possession but a last-ditch interception was required from Fernando in the 62nd minute, with Zamora poised to restore QPR's lead.
Edin Dzeko departed with an injury only four minutes after coming on as a substitute and City's misery appeared complete when Demichelis put Austin's cross into his own net under pressure from Zamora in the 76th minute, after Fernando had ceded possession to Fernando Vargas.
Dunne's centre-back partner Steven Caulker then nodded a header from replacement James Milner off the line and Green denied Toure in the 80th minute before Aguero got the second goal that his efforts, if not those of his team-mates, deserved.
