I have a regency hz42ste

During winter the venting pipes keeps icing

The flames are wispy blue dancing lifting off rocks and then fireplace goes out

Repair men from installer opens glass and fire place lights ... Keeps open for about 1 hr then replaces glass. Things work till it happens again.

The chimney is vertical about 50 feet straight up. It passes through cold attic to roof.

Dose anyone know how to keep this from happening.