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If your WiFi or even xDSL speeds suddenly fall, it'll be because of either radio interference or electrical noise (sometimes induced by the former). If you live in a block of flats you might see your speeds fall when everyone else gets home - multiple WiFi networks interfere with each other more when they are in use (there may also be contention issues but that's another subject - hopefully). Then again I've seen networks where the WiFi speed got knocked when central heating pumps kick in (old water pumps of whatever type are evil sources of electrical and RF interference).
You also have the Vodafone modem/router to contend with, it really is the special kid in the classroom, so I don't think anyone can actually tell you why the VF router goes wrong in the ways that it does!
What can be done? Maybe keep a diary, so you can see if there is any pattern, or give us a few more details so we can point you in the right direction.