So it sounds like this is a combination modem+router device.
If wired and wireless produce the same results, and you're using a combination device, there's really not much more you can do. Whatever issues you have are most likely the fault of the ISP. It's not unknown for ISPs to deliver less than promised, esp. after they have you as an established and otherwise happy customer. Things look good initially, but perhaps they start cutting back your internet access in order to serve more customers.
The only other test I would recommend (if possible) is using another desktop/laptop, just to eliminate the possibility it’s an issue w/ your current machine. If that’s not possible, you might grab a live Linux CD (e.g., Ubuntu), boot it (don’t worry, it won’t harm your current install, it runs completely in memory), and try from a different OS. If that suddenly worked, then it would suggest a problem w/ your PC (e.g., virus/malware).
Come to think of it, I’d perform an anti-virus/anti-malware scan as well. And make sure you're not running any *active* anti-virus/anti-malware products too (these can dramatically slow your speeds as they examine each and every packet for threats).
Anyway, if all the above checks out, it’s most likely the ISP. Tell them you tested it via wired and wireless, multiple machines, ran your scans, etc., no differences.