Could be cranes, but could also be the rollout of improved mobile phone services (4G services in the 800MHz range) which is happening around me in SE1. The site www.at800.tv provides more info. You might be affected if you're near a recently upgraded mast.
You could try an aerial amplifier. I bought one from Maplin for my Freeview box and have had no signal drop outs or artifacts since. They are mains powered and you can vary the gain - I keep mine on max.