I use code that looks like this to show a report in a Visualforce page. I've left out details that would get in the way:
<apex:page standardController="CustomOb__c" extensions="CobjController">
<apex:sectionHeader title="Manage Stuff"/>
<apex:form>
...
<apex:outputPanel id="MissingAttRpt" >
<apex:iframe src="/00O0000000AAJlV?isdtp=lt" scrolling="true" height="1588px" width="100%"/>
</apex:outputPanel>
...
</apex:form>
</apex:page>
The outputPanel is used so that the report can be re-rendered without refreshing the whole page.
Try getting rid of the showHeader and sidebar properties.