I argue 1986. This is when Gorbachev began staffing high level offices with large numbers of liberal communists and some outright anti-communists, and when his two chief allies -Shevardnadze and Yakovlev, came to prominence. 1986 was the year that Andrei Gromyko was retired from foreign affairs and replaced by Shevardnadze. Yakovlev, becoming head of the Central Committee's Department of Propaganda in late 1985, began in 1986 to staff the offices of universities, art unions, and media institutions with liberal-minded people, who began their campaign to destroy the public's faith in socialism, in their country and its history soon after, very carefully at first but growing in intensity as the decade came to a close.
In the economic sphere you are correct moonjosh. The 1987 Law on State Enterprise was the first step, giving managers more authority and attempting to decentralize planning. In 1988 the Law on Cooperatives allowed private business, much of it formed by former black marketeers that wanted to legalize their money. 1988 was also the crucial year that the Party secretariat lost its ability to coordinate the economy, resulting in economic chaos shortly thereafter.
I think it started with a combination of talks between Reagan & Gorbachev, and pressure from both the Polish Solidarity movement and later pressure from the Vatican. With a failing Soviet economy, Gorbachev hoped to do what his successors had done: wait out the storm. Pressure from the outside proved to be too much, and when Poland left, this opened the door for the Baltic states to dissent, and finally Perestroika was allowed to prevent a total and immediate collapse of society as well as the economy.
I think you've interpreted the question wrong comrade. He didn't ask "why" but "when". Anyway I don't think that pressure from Poland had much of a role in the USSR's internal dynamics at all. By 1985 the situation in Poland had stabilized, and some resented Walesa, either for his tendency to give in to pressure or for the strikes' disruptive effects on the Polish economy.
With regard to the Soviet economy, it was growing at 3-4% from 1983 on (up from 1.5% average between 1978 and 1982), due in large part to the disciplinary campaigns launched under Andropov, and to smaller modernization campaigns carried out in the fields of infrastructure and industry later on. The 1985 anti-alcohol campaign put a dent in the state budget, but up to the late 1980s growth indicators remained positive in most areas, disrupted, as noted above, by the removal of Party managerial oversight of economic activity.
Pressure from outside was only able to play any role in the Soviet collapse because of Gorbachev's personal vanity and his desire to be regarded by the West as a great and noble statesman. Besides, the ability and tendency of Baltic (and other) republics to dissent came as a result of the political and economic policies initiated by the Soviet government, not from pressure from Reagan, the Pope, or anyone else. In other words, it wasn't like Reagan said, "Mr. Gorbachev, tear down this wall" and Gorbachev, overwhelmed by Western pressure and internal dissent, fearfully said yes. It was more Gorbachev saying "Gee I'm a liberal that really doesn't like our socio-economic system, so I'm gonna haphazardly reform it more along Western social democratic lines, giving virulent anti-communists a dominant voice in society and shitting on our country's entire history. Oh and I really like it when I go to the West and everyone shouts my name ("Gorby, Gorby"), so I'm going to have to sell out all my allies in the Eastern bloc and the third world and hope that Reagan, Thatcher and Kohl help me to recognize the errors of my country's ways -oh and to give us money. "