Signal strength can be a bit misleading, what is also important is the signal to noise ratio within your environment (noise caused by obstacles, metal objects nearby, microwaves etc) and possible choking up of the available wireless bandwidth in your home and locality..... or maybe just a tired router that needs refreshing.
A few things that you can try initially.
Power down your wireless modem/router (pull the plug) and leave it for five minutes then plug back in. Update the firmware on the router. Check that there are no cordless phone base stations nearby or that no metal objects have recently been placed near the router itself. Count up all the devices using your router, the more there are on the same frequency the more likely that you will get dropouts.
Download a free app for example iStumbler, you can see the local routers in your area and what channel and frequency that they are transmitting on. If some are on the same channel then change yours or set your router to 'auto channel'. The other routers are likely on 2.4 gHz and unless you have a dual band router all your other devices are likely to be to as well, which may be slowing things up and causing dropouts.
