timestamp transmission accuracy

Hi, I am trying to run an application written in GNU Radio. The application insert tags e.g. "tx_time", "tx_sob" to control the transmission time. The value of "tx_time" is such that data is transmitted at start of new second. But I observe that the transmission and PPS is not synchronized when I see the transmission and PPS on scope. Sometime the transmission is late and sometime it is early. Has anyone faced this issue? Are there any examples that I can test. Thanks. -- Usman