Sounds like it's time to have the shutter cleaned and serviced. This is just routine maintenance, and slow speeds usually are the first to go off.

Meanwhile, if the subject is static, you can just take two 1/2 sec. exposures, if 1/2 sec is accurate.

Alternately, use the B setting and a metronome or a watch with a second hand. Set the metronome to 120 and subdivide the beat, so you're counting 4 beats per second, and you can easily time a 1 sec. bulb exposure accurately. Musicians do this all the time and are able to time motions not unlike pressing and releasing a shutter cable with an accuracy of a few thousandths of a second. I find it works well with times from 1/2 to 5 sec. More than that, and you can just look at the second hand on a watch.