iMac G5 wireless problem

After over a year of working fine, my imac G5 is suddenly having problems connecting to my wireless system. The signal is much weaker, but that may have to do with the repeater I use and the rainy weather. What is more perplexing is that it keeps dropping my home network (named default). When I click on the bars it no longer gives default as a choice and I have to add it manually. Another weird thing is that it suddenly gives options for computer to computer networks, something I have never tried to set up, and which was not there before. Anyone have any idea of why these weird things are happening? Anyone know how I can fix it?