Hi Hilary,
Welcome to the forums, though sad to hear that you are experiencing issues at particular times of the day. There is a lot of expertise around here who can and do offer assistance to users - they are users, customers just like yourself, especially when PN staff are not around on the forums, like for example out of office hours.
There are systematic approaches to sorting out (diagnosing) issues such as yours, one of which is the fault trouble-shooter - "We're way beyond that sort of messing about" does not help progress the resolution of your issue - there is a procedural process which rules out issues which from experience cause the vast majority of issues relating to variable performance. Not ruling them out before bringing in a BT Openreach engineer could result in you receiving hefty charges if the fault were to be found to be down to wiring on your premises.
You have not indicated if you are on an ADSL or fibre service, if ADSL, it is helpful to post your router stats, from both when you perceive things to be normal and at a time when they are not. I guess that you do not want to get involved in the technicalities of the matter and just want things fixed, however faults cannot be fixed if there is no fault being exhibited at the time someone looks. As likely as not, BTOR engineers are not going to be visiting you at the time the problem occurs. A bit of understanding of what could be happening on your side will therefore go a long way to helping sort this out.
How fast your broadband works is predicated on two factors: first how good is your line in general, this will determine your normal synch rate, that is how fast your line can transfer data; the second is how free your line is from interference (noise), this will determine the line's stability and ability to transfer data without error - the fewer errors, the less retransmissions required to move the data, therefore the overall speed is higher. Too fast a synch speed can lead to more errors and thus slower data transfer. The first task is to make your synch rate as good as it can be - the fault checker will identify any fundamental line fault before more complex issues are considered.
ADSL is quite susceptible to external noise - this is one reason (over and above heavy usage) why the effective speed of broadband can decrease at night time, directly due to the noise induced from MW radio transmissions. There are dozens of sources of noise which can have a similar or more marked effects, some which have been reported on the forums by users include microwaves, central heating boilers / timers, plasma TVs, fluorescent lights, laptop (or any other device) power packs, IR CCTV cameras, take-a-way shop extractor fans, street lighting - the source of the issue could be in your premises, it could be up the street. In the case of my personal experience, I had a combination of street lighting, IR CCTV cameras/ power supply, internal wiring and passing West Coast Main Line electric trains. All contributed to variable service and aside from the trains all were eventually identified and removed.
If your issue is related to REIN, there is no magic wand which someone can wave to make the problem go away, nor will changing ISP sort that problem out for you. there is though a lot of experience here to help you through the process. First of all, you need the basic checks performed on your line. This might identify a faulty joint, which when other nearby circuits are heavily used causes your line to pick up cross talk and this errors.
Posting your router stats as described here
http://community.plus.net/forum/index.php/topic,96155.0.html (if ADSL) would be a helpful action.
Are you predominantly using WiFi? Downloading and using inSSIDer will show you your WiFi network strength in comparison to others around you. Look at what is happening at the time you perceive the issues. Changing your channel might deliver some benefits.
I suggest that "something happens" in your locality at 4-5pm which then degrades some part of your network, either WiFi or broadband. This needs to be identified and then a resolution needs to be determined. If for example it turns out to be yours or a neighbour's plasma TV, there is nothing that PlusNet or BT can do to address that. At best you can get an electrical engineer to try to fix the issue, at worse you might have to get Ofcom or someone in to deal with interference to telegraphy services... lets hope this does not go that far!
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
