It does look suspicious to me. It's tough to put my finger on, but the phraseology doesn't look very 18th century. It doesn't sound like other Jefferson writing to me either. Also Jefferson is a rather conveniently famous and beloved figure to tag it onto if you aren't sure (or don't happen to like who really said it...)
With a fairly thorough googling, I did find rather a lot of instances of it attributed to Jefferson, but all IMHO from shaky sources, and none included a reference to the written material it was supposedly taken from.
As you said, I also managed to find it attributed to Adlai Stevenson. While I have the same suspicions as you that he could possibly have been re-quoting Jefferson, I did find at least one source for this attribution which is older than any Jefferson attribution I could find, and from someone with at least more credibility than a random internet schmoe: a memorial for Adlai Stevenson, penned by (then Vice President) Hubert Humphrey in July of 1969.
Perhaps my favorite words of Adlai were the ones I used often during
the 1968 Presidential Campaign: "Patriotism is not a short and
frenzied outburst of emotion, but the tranquil and steady dedication of
The word "frenzied" in particular looks very modern and anachronistic coming from Jefferson. Out of curiosity, I ran it through Ngram, and got the following:
In other words, it appears to have been a far more popular word in the early to mid 20th century (precisely when Adlai Stevenson was active) than in the mid to late 18th (when Jefferson was active).
So I'd go with Adlai. If you're wrong, at least you can blame a former VP for it, rather than a buch of random internet people you don't know.