Sounds like your DHCP pool is exhausted and when the clients refresh they are getting a new IP (though that's not how I understood DHCP leasing to work - as long as your assigned address was still valid, and you're renewing you should get the same one issued back to you - I suppose the DHCP server could be making issued addresses as now invalid).
What are you using to provide DHCP now?
DHCP clients normally try to verify their DHCP assigned address at the half way mark - 30 mins is half of the original 3600 lease time, so it makes sense that it's related, what doesn't make sense is why your machine is changing IPs?