As one who has been recently through this, I'll share my experience.
I have a late '70's Colnago Mexico. It was purchased about 10 years ago. Hadn't done much with it over the years except move it from place to place, due to workload, and other priorities.
The headset started getting stiff and I tore it down. I had some problems with getting the headset adjusted properly. The headset was Colnago, made by FT Bologna. As I understand it, these headsets had their problems. Small ball bearings, too.
I was also having difficulties with the threads on the steering tube. Sent it out to two different bike shops in the DC area, they wouldn't touch it, and finally sent it out to Franklin Frames in Ohio. Jack called and basically told me the threads had been chased one too many times sometime in the bicycle's life and he could repair it. Sent the fork out to him, took about a week and a few bucks, came back last week and was perfect. Serial number was still intact on the steering tube, Columbus logo was still there. He placed a sleeve in the steering tube and a new threaded section. Brazed in place, turned on a lathe to make everything concentric, good to go.
I also have a Mercier 300. Have had it since new in 1971. Although mine had Simplex dropouts. Yours may be newer.
Is this a plug for Franklin Frames? Maybe. The guy does great work. Can you find a French die block to chase those threads? I was going to get one to chase the threads on my Mexico but found out the die block cost about $150.
I don't have before and after pics, but there is a pic of the steering tube over on the framebuilding board.
If you have any questions, I'll be happy to help.
Hope this helps you,
Kurt