Poor WiFi > strong signal but poor/ intermittent ping

[#1]

Hi
I'm trying to help a friend and fellow PlusNet customer resolve their WiFi problems. They have a Thomson TG582n router and a couple of signal boosters around the house. The signal is quite strong, as you'd expect, but pinging the router is intermittent and mostly slow. I've tried taking the boosters out but that doesn't help. The house is 30/40 years old. The router is into the master socket in a small office to the side of the average sized house. I tried 3 laptops, an iPad and a tablet. They have a few phones connected and an xbox occasionally.
I've got an old Linksys router I can try, but it's not the best. Plugging directly into the Thomson works fine. They have a 6mbit connection.
Can anyone suggest anything I can try please?
Many thanks

[#2]

Has it always been a problem or has something changed?.
If the phones are not wired (dec phones)  try with them unplugged.
Maybe you can change the wireless channel?
What firmware do they have on the router?
Does the connection get any better if you take the laptop close to the router. but still use wireless?
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My sister in law had a router in the hall on a low table and the laptop only 8-9 feet away on the other side of a thin singlebrick wall again on a low table.
Problem seemd to be on each side of the wall there was a double finned central heating radiator that cut out most of the signal   - anything like that?

[#3]

Thanks cedlor
I think it's always been a problem. I've asked about dect phones, microwaves etc. They say there's none.
I've looked at the channels with WiFi scanner and there's is on channel 1, their strongest neighbour is on 11 and there's a weak signal on 5.
Their signal is very strong. Better than mine at home. I've tried standing right next to the router and the speed is still very slow, and drops off occasionally.
In their living room they're line of sight to the booster so again the signal is very strong... just slow!