Two polls on the independence question highlighted a 30-point difference of opinion, with polls that opened with questions about the performance of the SNP administration appearing to boost support for independence.
At the weekend, a poll gave the No side a 30% lead, prompting claims that with more than a year to go until the referendum the battle was all but over.
But yesterday the SNP hailed a survey by Panelbase that showed support for a Yes vote in the referendum has taken the lead for the first time since a TNS poll in The Herald in 2011.
SNP Depute Leader and Deputy First Minister Nicola Sturgeon said: "It is game on for next September's referendum.
"With just over a year to go, this poll indicates that the positive case for Yes is capturing people's imagination, while the No campaign's 'Project Fear' is running out of steam.
"The poll also shows that a clear majority of people back the Yes campaign's contention that Scotland could be a successful independent country, a common-sense proposition which the No campaign daily denigrates."
Professor John Curtice of Strathclyde University believes the key to the differences is the order in which questions are asked, rather than the particular sampling methods.
"How might we account for this seemingly surprising result?" he said, adding: "It would be interesting to see what happens if in its future polls Panelbase does what every other pollster has in fact been doing and that is asking referendum voting intention first in advance of any other questions on the subject at all.
"Given the continued popularity of the SNP when people are asked how they would vote in a Scottish Parliament election, perhaps having that question in front helps explain in part at least why Panelbase's polls are always making the best reading for the Yes side."
The SNP analysis showed that support for a Yes vote had increased by seven points since the last Panelbase poll in July for the Sunday Times, as undecided voters appear to be opting for Yes. Support for No fell by three points.
The detailed breakdown showed that 24% of people who voted Labour in the Scottish Parliament constituency vote in 2011 intend to vote Yes.
Among women aged between 35 and 54 years old, Yes leads No by 45% to 41%.
The poll shows that 93% of Yes supporters are very likely to vote in the referendum, compared to 88% of No supporters.
Professor Curtice concludes: "There is good reason to believe that the particularly favourable result for the Yes side is a consequence of the way in which the poll was conducted.
"That, however, is not to say it should be ignored. Evidently if the Yes side could persuade people that independence would deliver a successful country free of unwanted Westminster rule, then they might well drive people into their camp. But we should not as yet presume that they have suddenly made significant progress in actually achieving that task."