Windows 7 - Java JRE 6 not using cache in Windows 7 results in poor performance

I am using a commercial software package that uses Java. Within the company all XP users work fine and performance is great, however for all Windows 7 users the performance is appalling. After weeks of testing with the software vendor it appears that under Windows 7 JRE is not using any of the cashed images and objects and is going off to the server all the time. This can be seen in the Java trace files. This behaviour is not seen on the XP clients. The windows 7 PCs are taking about 4 times as long as the XP clients to load a page. The Java control panel program shows that the items have been succesfully cached locally on the windows 7 PCs but are not being read at run time.

We have tried...

deleting the cache
lowering windows 7 security, UAC off
lowering ie9 security
even removing fonts

without any success. The vendor has now almost given up blaming a 'network policy' within our company infrastructure. Whilst I realise Windows 7 security is a lot more advanced that XP I am not aware of any network, firewall or group policies that say to Java, "Hey don't use your cache". I may be wrong.

Does anyone have any further suggestions of where to look?