How to make sure unstable/ slow connection is due to ISP, not anything on my side

Solution 1:

First, you must be sure, that there is your ISP fault.

Try mtr utility. It is combination of traceroute and ping which can provide you more information about latency bottlenecks. From its output you will find out, where the problem occurs.

Simple usage example:

# mtr -r -c 10 site.com 

Then, if the problem is with ISP, send them the output.

Solution 2:

mtr is the tool you need.

mtr combines the functionality of the 'traceroute' and 'ping' programs in a single network diagnostic tool.

As mtr starts, it investigates the network connection between the host mtr runs on and a user-specified destination host. After it determines the address of each network hop between the machines, it sends a sequence ICMP ECHO requests to each one to determine the quality of the link to each machine. As it does this, it prints running statistics about each machine.

Solution 3:

I think your case is lost, although it may depend on who is your ISP.

In your agreement there is no such point that something is guaranteed. If it's working (lousy but it is) they probably won't give a broken nickel. The least thing you can do is to phone them and tell them exactly what you said in here. And ask them politely to run similar tests on their side.