It may be in the cold, but heating tends to release smells easier. Don't know if this may help, but sometimes changing the anode in the WH can help. I'd check with the local plumbing supply house and ask them what the pros use in your area then change it if it's a common problem.
How hot do you run the WH? Have you tried cranking it up for say overnight? That would tend to kill anything that may be biological.