Very inconsistent internet speeds?

Hi

recently my wifi and Ethernet speeds on all my computers have been horridly inconsistent. Sometimes ill get the normal 15mbps (what i pay for from telstra), and quite often now i get speeds between 0.2 mbps and 0.5 mbps, with it sometimes dropping out altogether. I have been able to replicate the problem on 2 computers an iPad and an iPhone. Then 15 minutes later ill be back on 15 mbps, i get jump on the couch to watch some you tube and bam im back to 0.2 mbps

Please Help

Thanks

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