异步下载UITableView时出现问题

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @\"Cell\";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier] autorelease];
    }

    SearchObject *so = (SearchObject *)[_tableData objectAtIndex:indexPath.row];
    cell.textLabel.text = [[[[so tweet] stringByReplacingOccurrencesOfString:@\"&quot;\" withString:@\"\\\"\"] stringByReplacingOccurrencesOfString:@\"&lt;\" withString:@\"<\"] stringByReplacingOccurrencesOfString:@\"&gt;\" withString:@\">\"];
    cell.detailTextLabel.text = [so fromUser];
    if (cell.imageView.image == nil) {
        NSURLRequest *req = [NSURLRequest requestWithURL:[NSURL URLWithString:[so userProfileImageURL]]];
        NSURLConnection *conn = [NSURLConnection connectionWithRequest:req delegate:self];
        [conn start];
    }
    if ([_cellImages count] > indexPath.row) {
        cell.imageView.image = [UIImage imageWithData:[_cellImages objectAtIndex:indexPath.row]];
    }
    return cell;
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [_cellData appendData:data];
    [_cellImages addObject:_cellData];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    [self.tableView reloadData];
}