Did you complete a leakdown & compression test on the motor ?
The car never overheated or smoked white under load ?

If the compression/leakdown results were fine I would resurface the cylinder head and install an oem headgasket using coperspray.

I would then run 100% distilled water in the coolant system to monitor the color of the coolant.

If the issue still continues I would consider throwing a wideband on the car to see if there are any issues with fuel delivery etc.