Geostationary satellites which have gone out of commission for one reason or another will often be tumbling in their orbits, hence the pulsing. Also, at the right time of year, geostationary satellites can flare, making them visible temporarily with the naked eye.
Here is an interesting page:
http://homepage.usask.ca/~ges125/rasc/Geosyncs.html
As an item of note, due to geometry, geostationary satellites tend to plague M42 for many northern hemisphere photographers. For us they shift about 10 degrees north, hanging out around +4 to +6 degrees north declination (see
http://www.claysturner.com/dsp/astro1.pdf )
We had one geostationary visible at QLD Astrofest this year on at least 3 successive nights, flaring visually about every 15 seconds for about 15-20 minutes.
Regards,
Eric