I live in a 1920's era house, and have probably the same system you have. I had the same problem, but changed all the air/steam valve vents, and it seems tohave gone away. It's been over a year, so I assume changing the vents corrected it.
I did notice one of the tenants in theis house had previously installed some sort of manual valve instead of an actual correct radiator valve.
Harry, thanks alot, your site has some great tips I can use!