php 5 and mysql failure

I'm using a php script which performs three xml queries to other three 
servers to retrieve a set of ids and after I do a query to mysql of the 
kind
SELECT * FROM table WHERE id IN ('set of ids');
Although I'm sure the connection to the database is ok, I sometimes get 
an error of this kind:
*Warning*: mysql_fetch_object(): supplied argument is not a valid MySQL 
result resource in ...
This does not happen every time i run the script, only sometimes.
If I echo the query, copy and paste in phpmyadmin, or if I perform the 
same query in a script that does only the query without the rest it works!
After troubleshooting this issue I noticed that it usually failed when I 
had a big set of ids (positive response from more than one server). This 
means that the script used a bigger amount of memory and probably more 
resources, but I did not get an "out of memory error", I got the one 
described bfore.
My question is, is there any kind of limit somewhere in php5 or in mysql?
Thanks for help,

YEHUDI GARRETT