35 ford coolant loss?

[jack99]

I have a 1935 Ford 85 Hp v-8. I am at a loss as to why I keep loosing radiator water. I do not see any spillover on floor of garage, nor do I see any steam from tail pipe. All of a sudden I see a high temp on temp gage and I have to add another gallon to radiator. All the time the engine runs like a clock. What do you think ? Bad head gasket? Takes a week or two to present the problem. Any advise or thought would be appreciated. Thanks!