|One of the tools we've been using to monitor bandwidth is "Cacti Server"|
This software allows you to monitor specific ports on switches. This gives you some flexibility and can help you figure out if the problem is your network (a bottleneck somewhere) or if it's your provider.
Because you say "The problem is our internet connections gets very slow very often in a day" I'm going to suggest the issue is likely a user issue. Should monitoring show you have no real bottlenecks, and it's not the provider, then the next most likely candidate is user abuse of available bandwidth.
If you have 180 people and half of them are doing bandwidth intensive things like streaming videos (ie: watching YouTube) or listening to radio stations, or even possibly downloading large quantities of files, it can all degrade overall bandwidth.
There are tools that allow you monitor what kind of traffic is going through and even stop it. We have a device called a "packetshaper" made by Packeteer, that allows us to monitor and restrict particular websites, and types of traffic. For instance, I can completely cut off FaceBook, YouTube, BitTorrents, etc etc etc. (It's a very comprehensive device and the list is quite long)
If you find the issue is user abuse, get together with the big boss, write out an "Appropriate Use of Network Resources" policy, have all employee's read and sign it. Then watch and the first person you catch doing something listed as a no-no on the policy, fire them immediately and you'll be surprised how quickly your problem goes away.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.