rad overflow bottle issues...

[soaper]

my '95 civic is developing a coolant problem. seems the coolant flows into the overflow bottle but does not return to the rad when the engine cools down. i have changed the rad cap and do not notice any apparent leaks of coolant anywhere. the temperature gauge reports that the engine is fine and not over heating - i've been emptying the overflow and refilling the rad every 3 days. thermostat? head gasket?

any help would be appreciated.

soaper.