Perception isn't always reality. Just the stats, no game film review.
1st qtr 32 of 53 for 416 yards, 60% 4 TD 4 INT 136.13 passer rating
2nd qtr 43 of 76 for 499 yards 56.6% 2 TD 2 INT 115.16 passer rating
3rd qtr 33 of 61 for 502 yards 54% 5 TD 1 INT 147.0 passer rating
4th qtr 30 of 62 for 347 yards 48% 2 TD 4 INT 93.15 passer rating
This shows me that Nebraska is able to game plan for the 1st quarter, make some adjustments at halftime, but does not seem to be able to adjust to the adjustments of the defense on the fly.
Stats when losing by 8-14 points
34 of 69 for 408 yards 49% 2 TD 1 INT 105.62 passer rating
Stats when losing by 15+
31 of 55 for 443 yards 56% 4 TD 2 INT 140.75 passer rating.
This seems to support your theory that he is playing better when the game is no longer in doubt.
However when Nebraska is in 3rd and 7+ yards to go
20 for 45 for 222 yards 44% 2 TD and 4 INT 80.14 passer rating.
In conclusion, I would say that you are partially correct, Lee is has better numbers when the game is no longer in doubt, however, when it is an obvious passing down, he struggles.