The same as every other post in this section, the network can no longer cope, you'll be classed as being in a 'high utilisation' area, basically meaning theres too many people signed up to VM in your area and the network can't cope with the demand.
Hopefully a mod will reply and give you a fault code for the area and a fix date where it will get pushed back over and over while they spend money sponsoring football instead.
If your speed only drops in the evening (peak hours) and you can reliably achieved the full speed during the day then there's nothing mods can do really. It's contention/peak utilisation issue. Either VM increased the bandwidth for your area or more people leave thus less people using the internet during peak hours.
I've located your account and I can see an issue with the network in your area affecting speeds at peak times - Ref.(F003834674). Our technicians are working to get this resolved and have set a review date of 26th April.
I'm really sorry for any inconvenience this may cause.
Tech fan? Have you read our Digital life blog yet? Check it out