Thank you for your interest and prompt reply.
Previously I had ISP modem>airport router>computer CPU. Problems with downloads occurred only after several months.
Router was (according to ISP) interfering with internet access; instructed to power off router, then power on again. This procedure was carried out several times; surprisingly, the wifi service in my house was apparently unaffected (????). Would have thought that router settings would have been deleted upon power loss.
Anyway, when Airport Extreme router was removed from system, file downloads returned to usual.
It appears that this Apple router interferes with downloads of large files. Surprisingly, the transfer capabilities of this router are not mentioned on the appropriate Apple webpage, unlike PC routers.
Anyway, I will try splitting the ethernet output from my ISP modem between my computer CPU and this Airport router. Do you think the signal to either of these will suffer signal degradation due to this connection? Unlikely since it is a passive device.
I now must locate a local retailer who sells this ethernet splitter.
Thanx again for your input!