wireless problems in 10.5.8

Hi

I have an imac running OSX 10.5.8

powerpcG5 1.8Ghz with 2 Gb RAM

Internet was running fine until this week. I use DSL from Verizon with wireless router.

My Windows 7 laptop runs fine- speed test ping 59 download 2.85 upload 0.66

My imac is extremely variable but mostly very slow:

ping ranges from 60 to 157ms

download ranges from so slow I get an error message to 2.69
upload ranges from .39 to .61

what do I need to do to get it working again?

is airport shot?

thanks,

Eric