Need advice regarding race condition

Hi all,
 
I'm currently debugging a ship monitoring system. In that application I found whole heaps of bugs, fortunately I can solve every single one of them, except this one. So please help me.
 
Here's the explanation:
The bug is located within a distribution server, and I suspect (No, I'm sure of it) that there's an extreme race condition between the receiver class (will be addressed as Receiver) and sender class (will be addressed as Sender).
 
Distribution server works like this:
1. Receiver receive data.
2. Receiver write data into a data storage class (will be addressed as DataStorage).
3. Sender check whether DataStorage is empty or not.
4. If DataStorage is not empty, Sender will proceed to send the data.
Or so they think.
 
Problems I found so far:
1. A guaranteed race condition 'cause Sender check using a flag.
Okay, I've applied a lock into Sender.
2. I can't safely release Sender's lock.
Why? it's simple, really. It's because receiver works as interface. It receive data when onDataReceived method is called by a jar file (it's called QuickServer).
 
So here I am, looking for some advice regarding this matter. The only solution I've found so far is:
1. Release Sender's lock when a new data is received.
2. Sender will raise a flag to suspend any Receiver's incoming data.
3. Sender wait for any unfinished operation by Receiver (Thread.sleep(500)?).
4. Sender grab all data.
5. Sender pull down the flag.
6. Receiver start it's suspended operation, Sender process and send the data.
7. Everyone live in harmony till Sender's lock is re-applied after sending data therefore go back to step1.
 
[edit]My solution failed,[/edit]Is there any other solution? I'm a fresh graduate from a vocational high school therefore I'm looking for some experienced expert's solution.
 

Thanks in advance,
 
Firo A.V.