Have you try with the 1.8.2 release of AjaxSearch ?
This last release has solved lot of issues.
=================== ISSUES CORRECTED ==============================
==== AJAXSEARCH-6 : Sort with order and ranking parameters. Results with a wrong order
http://svn.modxcms.com/jira/browse/AJAXSEARCH-6
==== AJAXSEARCH-12 : Take into account of the &advSearch parameter when the &minChars is checked
http://svn.modxcms.com/jira/browse/AJAXSEARCH-12
AJAXSEARCH-13 : Wrong Highlighting of search terms
http://svn.modxcms.com/jira/browse/AJAXSEARCH-13
==== AJAXSEARCH-15 : use $modx->getFullTableName(’’)
http://svn.modxcms.com/jira/browse/AJAXSEARCH-15
==== AJAXSEARCH-17 : Livesearch - start of the search only when the minimum character is reached
http://svn.modxcms.com/jira/browse/AJAXSEARCH-17
==== AJAXSEARCH-18 : function stripHtml incorrectly works
http://svn.modxcms.com/jira/browse/AJAXSEARCH-18
==== AJAXSEARCH-19 : Parse error if the user enters two or more spaces between words in the query
http://svn.modxcms.com/jira/browse/AJAXSEARCH-19
==== AJAXSEARCH-21 : search for something like "ecuador span"
http://svn.modxcms.com/jira/browse/AJAXSEARCH-21
==== AJAXSEARCH-22 : you get a TV value with tvPHX, only if the search has found your term in the TV
http://svn.modxcms.com/jira/browse/AJAXSEARCH-22
==== AJAXSEARCH-24 : AjaxSearch stops searching through TV content after a certain number of characters
http://svn.modxcms.com/jira/browse/AJAXSEARCH-24
==== AJAXSEARCH-25 : Security issue with AS_offset
http://svn.modxcms.com/jira/browse/AJAXSEARCH-25
==== AJAXSEARCH-26 : If the offset exceeds the number of documents retrieved, modx crashes
http://svn.modxcms.com/jira/browse/AJAXSEARCH-26
==== AJAXSEARCH-27 : AjaxSearch and Document Group permissions
http://svn.modxcms.com/jira/browse/AJAXSEARCH-27
==== AJAXSEARCH-29 : When using french UTF8 or english UTF8, I get a blank line at the top of the search
http://svn.modxcms.com/jira/browse/AJAXSEARCH-29
==== AJAXSEARCH-30 : htmlspecialchars() expects at most 3 parameters, 4 given
http://svn.modxcms.com/jira/browse/AJAXSEARCH-30
==== AJAXSEARCH-31 : parse error with AjaxSearch 1.8.2 when the search has no results and filter parameter used
http://svn.modxcms.com/jira/browse/AJAXSEARCH-31
==== AJAXSEARCH-32 : pages not to validate as XHTML
http://svn.modxcms.com/jira/browse/AJAXSEARCH-32