Back in an E46

Just bought a 05 330ci ZHP with 92,000 miles after finally getting money from insurance after my 03 325ci was totalled, car is clean and everything appeared fine but driving it tonight for the first time I had the onboard temp display on after it seemed like it was taking forever to warm up. What I saw has me worried, after several miles it was up to 70 at stoplights but would actually drop as low as 64 when driving! Is this just a bad thermostat or could it be other issues that are more serious? I can't think of anything but thermostat that would cause this. Anyone who knows your help will be appreciated.

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Back in an E46