Fel Pro thermostat gaskets?
Thermostat housings can be a problem
"Back in the day" when housings were good, machined, heavy cast iron, there WAS no problem
Then "Cal China Chrome Custom" came along. And OEM alloy housings. And those silly stamped steel ones
As stated above, always check for flatness
Be careful not to overtighten so as not to warp the housing
Don't use ANY sealer that will act as a lubricant 'slime' and allow the gasket to waltz out of place
Don't overtighten
Be careful you do not overtighten
Be VERY careful not to overtighten