One possibility is that the new covering that was applied doesn't allow heat to be conducted away from the resistor as efficiently as the original enclosure, and the heat is essentially trapped. Another possibility is that the new covering isn't able to withstand the temperatures that the resistor normally reaches under high volume conditions. A third possibility is that if the resistor was resoldered when the work was done, it was damaged internally by the heat from the soldering iron or gun.
Regards,
-- Al
Regards,
-- Al