I had a similar problem a few weeks back, changed the splitter and the wire, made no difference, I ended ip changing the modem and that fixed the problem, must have been a dodgy modem even though it had only been used for a few months.
Can you get a spare modem from anywhere to try it out? Vodafone were not very helpful I have to say, I thought they should replace the modem but they wouldn't , so I would have had to buy a new one except I was able to dig out a spare one and that worked. It can depend on who you get in Vodafone tho when you call, sometimes on other things they have been very helpful