SOFIA (Reuters) -Bulgarians head to the polls on Sunday for their sixth parliamentary election in three years, but analysts say the vote is unlikely to yield a stable government that can end prolonged political instability and unblock economic reforms. Bulgaria, the poorest member of the European Union, has been plagued by revolving-door governments since anti-corruption protests in 2020, with a series of elections producing shaky coalitions that swiftly crumbled. The latest opinion polls suggest no party will win a majority, setting the stage for a new round of coalition talks once the votes are in.