I have revised herein my question of Aug 18 and update my research based on the most helpful suggestions of Peter Schor and tchrist of Aug 18, 2013.
I'm not a Cervantista and don't speak Spanish. I've just been fond of this phrase for a long time and, in opinion, it still remains the most pleasing, concise description of the loveable Don Quixote.
Cervantes wrote Part I in 1605 and 10 years later in 1615 Part II. This quote comes from his Part II Chapter XVIII (2:18.)
The exact quote is not found in any of the major translations which I list below. The exact quote actually appeared first in Bartlett's Familiar Quotations 14th Ed. 1968, Emily Morison Beck, Editor, p. 196. Footnote 3 says the editors used for reference Motteux's translation published by the Modern Library Giant edition, New York 1930. I just now bought my copy. Of the 114 Quixote quotes, 112 (97%) are verbatim quotes from the book, only four are different. Interestingly, one of the four is our quote in question.
So, who translated this quote? As Peter Schor has so kindly contributed, "It's possible Bartlett...translated the Spanish themselves..." I'm also greatly indebted to tchrist, "The origin of the choice of translation lies always with the translator...is all open to the translator's choice of nuance..."
1620 Thomas Shelton, first English translator, He is a curious madman, and hath neat dilemmas
1712 Peter Motteux, 1719 Ozell's revision of Motteux's translation, Modern Library Giant Ed. (New York 1930) p. 557, He is Mad past Recovery, but yet he has lucid intervals
1742 Charles Jarvis, Vol IV p. 129, His distraction is a medley full of lucid intervals
1755 Tobias Smollett, Modern Library 2004, p. 366, he is a party-coloured maniack, full of lucid intervals
1881 Alexander J. Duffield, p.76, he is a mingled madman, of many lucid intervals
1885 John Ormsby, 1981 Douglas/Jones revision sans lenghty notes, hence ODJ, he is a madman full of streaks, full of lucid intervals
1888 Henry Edward Watts, Vol IV p. 209, He is a fool interlarded.1---full of lucid streaks. Footnote 1: Un entreverado loco. Entreverado is generally applied to bacon, meaning fat and lean commingled. See Donoso the poeta entreverado, in the prefatory verses to Part I. (Vol II. P.26)
1949 Samuel Putnam, Modern Library p. 624, He is a streaked madman, full of lucid intervals.
1950 J. M. Cohen, Penguin Classics, Collector's Library reprint 2008, p. 625, He is mad in patches, full of lucid intervals
1957 Walter Starkie, Signet Classics p. 652, He is mad in patches, full of lucid intervals
1968 Bartlett's Familiar Quotations 14th Ed. 1968, p. 196, He's a muddled fool, full of lucid intervals, III, 18, p. 556. P. 193 footnote 3: Translated in 1700-1703 by Peter Anthony Motteux (1660-1718). Page numbers are those of the Modern Library Giant edition. (note 2 typos: Motteux was born 1663, correct page is 557)
1995 Burton Raffel, Norton Critical Edition p. 453, he's plainly an off-again on-again madman, shot full of lucid intervals
1999 Oxford Dictionary of Quotations 1999, p. 200, He's a muddle-headed fool, with frequent lucid intervals
2000 John Rutherford, Penguin Classics p. 604, he's mad in streaks, complete with lucid intervals
2003 Edith Grossman, HarperCollins p. 511, he is a combination madman who has many lucid intervals
2005 Tom Lathrop, Signet Classics 2011, p. 637, He's a crazy man with periods of lucidity
2006 James Montgomery, Hackett Pub. Co. I don't have
The old Italian proverb, Traduttore, traditore, did warn that interpreting translation phrases can be treacherous.