Listen to this guy. I work in the industry, too, for a regional ISP in a very rural area, and I have a couple of things to add.
To begin with, I know it hurts to hear this, but sometimes reality bites: the residential ISP business model is BASED on oversubscription. Period. Anybody else who tells you otherwise is lying or doesn't know what they are talking about. When an ISP sells a residential or SMB customer a 3Mbit/s down asynchronous connection at under $100/mo, it's guaranteed they don't have the bandwidth to back this up for you and everybody else they have sold a connection to. All of the usage models for scaling up bandwidth are based on bursty usage by their customers. They simply cannot afford to have every single customer of theirs pulling down their 3 megs all simultaneously, 24 hours a day, 7 days a week. Thus all of the "up to" language that was surely a part of your "contracted" rate.
But thanks to recent inventions such as Bittorrent and Netflix, certain customers *are* constantly filling their pipe 24/7. And people wonder why ISPs are in such a hurry to institute pay-per-use models...I mean, what other industry that resells scarce, shared-resource services sells it to you at flat-rate all-you-can-eat pricing? Electricity? Water? Telephone? Fuel? None of those.
Now, it absolutely could be argued that if you are seeing 100-200kbit/s down at certain times of the day, they aren't keeping up their end of the bargain because they haven't scaled up their upstream bandwidth to cope with increased demand (especially if they are continuing to install new customers). A successful last-mile ISP will be watching their usage, constantly running the numbers, and making sure that they still have enough capacity to meet demand at any given time. Of course, this is all still done within the assumption of "bursty" usage models, so if they have 10 customers each provisioned at 3Mbit/s down, their models are not going to suggest to them that they need to have 30Mbit/s of total capacity available. So if all of those customers are filling their connections 24/7, then that creates a problem.
And the problem is a real economic problem. The previous poster was correct in saying that the ISP is not making money off of you hand-over-fist. If they are strictly an ISP, I guarantee you they are barely squeaking by. (If they are a regulated incumbent telco with an ISP side-business, like a Verizon or Frontier or CenturyLink, that's another whole story...) ESPECIALLY if they are a rural ISP. Verizon/Frontier sells, what, 3Mbit/s DSL for around $30/mo to residential users? That's great. I *guarantee* you that IF we are talking about a rural ISP, they are LUCKY if they are paying a rate of $30-per-MEG-per-month to their upstream. That would be CHEAP. And you expect them to turn around and sell you a 3 MEG all-you-can-eat circuit for $30/mo? That would mean you are paying them 1/3rd of what that kind of bandwidth actually costs them to get for you. That's called a money-losing proposition.
So, to the OP: by all means, complain to your ISP. For all we know, your problems are not related to constrained throughput as a result of peak usage, and are instead being caused by a physical problem with your circuit. But keep in mind that there's a reason that to this day, getting a connection with an actual contracted SLA is not cheap. There's a reason why you can still find yourself paying $300-500/mo or more to a telco for a T1 (~1.5Mbit/s synchronous) circuit where that throughput is guaranteed. If you actually need 3 megs down guaranteed to you 24/7, then you're going to have to pay dearly for it.
The problem is that nobody is willing to pay what it actually costs.