you have 2 probable problems - range and congestion.
I had a set up where I wanted to restrict access times to stop the kids accessing the internet after a certain time of night, so had a set up with the ISP based modem/router with WiFi turned off and a DDWRT based router just attached to this to provide the WiFi in the home, but this was just wireless G, and while coverage in the home was fine, the devices using this started to multiply as 6 people started to use more devices (PC's, tablets, phones) to the point where the router started to fail to be able to register new connection attempts.
I think I'd recommend trying a powerline adaptor with WiFi access - this would create an AP (Access Point) separate to your ISP router (with a separate SSID - network identifier) that ultimately plumbs into an Ethernet connection on your ISP router - this pushes some of the traffic off the ISP router's wireless connection, which should make access there easier for the ones still using it. My only note of caution would be if the electrics are old, & the circuits are separated out as this might prevent the the base powerline unit communicating with the extender - which would rather negate the point.
If you're worried about the electrics, then the other best option would be to a basic wired Ethernet connection - you could tack something like that (the cabling) the the skirting board & it not be too obtrusive & then pop a TP-Link WDR3500/3600 on the end (we've found them a good balance of price to functionality at work
) on the uplink port thereon. This would leave you with a couple of potential Ethernet ports (if you feel like doing some more wiring) to use with... say an Xbox in someone's room) and and a further wireless connection being distributed (and providing more potential WiFi connectivity).