If Virgin Media have a managerial level I'm sorry to say they are entirely detached from operational issues.
If technical staff have visited and have not found any actual faults then it's likely that VM have made too many user connections for the local equipment capacity.
You can check if any fault is known by phoning 0800 561 0061.
An idea of local capacity overloading can be gained by setting up a free quality monitor at https://www.thinkbroadband.com/broadband/monitoring/quality and posting a Live Link to this thread.
Ultimately you may just have to find a more reliable supplier.