sudden slow download speed

My iMac download speed has slowed down to less than 5mb/s. This started suddenly about a week ago, after functioning fine for years, and has increased in severity over the week i.e. from noticeable slowing to almost a dead-halt (blank screen when trying to open emails/download attachments). I don't know if this is relevant but about the time it started, there's a console message showing this: Safari [140]<Error>kCGErrorIllegalArgument:CGSFlusingWindowContentRegion:Invalid window Seems this started at 00:00:00 a.m. and repeated about a squillion times for 9 hours, followed by batches of other multiple-repeated messages, including the same as above but with Safari[144] in place of the 140 code.

Meanwhile another imac, ipad mini and pc are running normally off the same system, with Virgin as ISP connecting through a mac wireless router. I've tried switching off and restarting the system as well as the iMac.

Online support from Apple helped me clear the cached stuff, and it seemed much sprightlier but then after some internet/yahoo use it slowed again. I took it to a mac store where the location was reset and it tested at 25 mb/s, but then back home after logging in to yahoo it has returned to <5. Could yahoo be doing something specific? And why do none of these factors - the ISP, router, programmes accessed online - affect the functioning of the other devices operating under the same conditions? Any ideas?