Blank Desktop Screen After a few logins

Blank Desktop Screen After a few logins

Hi,
I have purchased the Wyse Pocketcloud for Android a few months back. Initially when I use the pocketcloud, there is no problem. But quite recently, after I did the version update of the Pocketcloud when my phone automatically prompt me, I encounter the following problem. At the start of the day when I use my phone to access my desktop, there is no problem. But after I login and out a few times, the desktop screen in my phone starts to turn black or blank. The only way to solve this problem is for me to get back home at the end of the day to restart my desktop again. I am accessing my desktop using 3G network. The following is my configuration

My Desktop:
1. Windows Vista Home Premium (64bit)
2. Wyse PocketCloud Windows Companion (I have kept it updated)
3. UltraVNC
4. CPU - Intel Core2 Quad CPU Q9400

My mobile:
1. Samsung Galaxy Tab (7 inch) GT-P1000
2. Android Froyo
3. Wyse Pocketcloud Pro

Please help.
Thanks!!

Regards,
Chmar