Loosing internet connection. Modom or router?

[dooopo]

Cable guy cam over and said im getting good signal. Today I did get a pop up message saying somtehing like "IP adress conflict with other system on the network" or something like that.

I can get internet off and on but some days get no signal all day. Today it is off and on like 5 mins I get connection and then after that it takes hours before I get connecting again. I keep unplugging the router and modom like the guy said to do but it only works some times and for only minutes at a time I get connection.
I just hooked the modem up to the computer and it does the same thing so now I know it's not the router. So is this just a bad modem or my computer?