SQL Server 2008 Containstable generate negative rank with weighted_term
Qualifier: Since I can't recreate this, I'm unable to know for sure if this will fix the problem.  However, these are some things that I'm seeing.

First off, the ampersand, pound sign, and semicolon are word-break characters.  That means, that instead of searching for the string "p&amp;#233;toncle", what you're actually searching for is "p", "233", and "toncle".  Clearly, that's not your intent.  

I have to presume that you have the text "p&amp;#233;toncle" somewhere in your dataset.  That means you need that entire string to be complete.

There are a few things you can do.  

1)  Turn off Stopwords all together.  You can do that by altering the full text index to turn it off.

Note that you have to have your database set to SQL Server 2008 compatability for this to not generate a syntax error:

ALTER FULLTEXT INDEX ON Table1 SET STOPLIST OFF;


2) Create a new stoplist

If you create an empty StopList, you might be able to add the stopwords that you want or copy the system stoplist and remove the stopwords that you don't want.  (I would advise the second approach). 

Having said that, I wasn't able to find the &amp; or # in the system stoplist, so they may be hard coded.  You may have to simply turn the stoplist off.

3) Change your search to ignore the "p&amp;#233;toncle" case.

If you drop the "p&amp;#233;toncle" from the ISABOUT and change them to "p toncle", it might work:

'ISABOUT("pétoncle" weight (.8), "p toncle" weight (.8))'


Those are just some ideas.  Like I said, without being able to access the system or recreate the scenario, we won't be able to help much.



Some more information for your researching pleasure:


Stopwords and Stoplists
Alter Fulltext Index syntax
FullText search using Thesaurus file and special characters