PHP crash - vb query

[Valof]

Hello!

I have very important problem. When a lot of logged users are viewing my site - my web hosting developer blocks my site because
"my site makes php loop and then crash"
that's query with problem is:

poll([{fd=4, events=POLLIN|POLLPRI}], 1, 0) = 0 (Timeout)
write(4, "m\0\0\0\3\r\n\t\t\tSELECT * FROM user FORCE INDEX (lastactivity)\r\n\t\t\tWHERE lastactivity > 1272232800 ORDER BY username\r\n\t\t"..., 113)

brk(0x911b000) = 0x911b000
time(NULL) = 1272306532
time(NULL) = 1272306532
stat64("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2679, ...}) = 0
time(NULL) = 1272306532
time(NULL) = 1272306532
stat64("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2679, ...}) = 0
time(NULL) = 1272306532
time(NULL) = 1272306532
stat64("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2679, ...}) = 0
time(NULL) = 1272306532
time(NULL) = 1272306532
stat64("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2679, ...}) = 0
time(NULL) = 1272306532
time(NULL) = 1272306532

write(4, "\242\1\0\0\3\n\t\tSELECT\n\t\t\tuser.username , (user.options & 512) AS invisible, user.usergroupid,\n\t\t\tsession.userid, session.inforum, session.lastactivity, session.badlocation,\n\t\t\tIF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid, infractiongroupid\n\t\t\t\n\t\tFROM session AS session\n\t\tLEFT JOIN user AS user ON(user.userid = session.userid)\n\t\t\n\t\tWHERE session.lastactivity > 1272305629\n\t\t\t\n\t\tORDER BY username ASC\n\t"..., 422) = 422

Queries are from file (developer said):

"/home/login/domains/nazwa-domeny.pl/public_html/includes/functions_forumlist.php"

and second (i blocked/uninstalled all plugins which show any users list etc, so query has been changed):
Deleted plugins:
-advanced statistic
-members who has visited
-guest tracking

write(4, "\307\0\0\0\3\n\t\t\t\tSELECT forum.forumid, lastpost, lastposter, lastthread, lastthreadid, lasticonid, threadcount, replycount, lastpostid, lastprefixid, default_tags_ids\n\t\t\t\t\t\n\t\t\t\tFROM forum AS forum\n\t\t\t\t\n\t\t\t\t\n\t\t\t"..., 203) = 203

Please help me!

Best regards