bill001g :
Run a traceroute to some location.
Then ping the first 2 hops in the trace with a continuous ping.
The first should be your router in your house. The second will likely be the ISP router at their nearest office. Make sure you do this on a ethernet cable. If you really only have a modem and your PC is getting the actual IP directly then the first hop will the ISP router.
If you see issues in the first hop this indicates a issue with your router. It is kinda rare to see issues here.
The second hop is where most problems occur. Most the time it is a circuit problem but it can be your modem or the cables in your house. The ISP can test the circuit and tell you. Of course you can just replace the modem so when they try to blame it you can say you already replaced it.
If it is in hops past the first 2 you are in the ISP network or maybe even another ISP network. If the ISP are not a bunch of dummies they might be able to troubleshoot this.....but I wouldn't get my hopes up.
Believe me i think they are dummies, I've explain the situation on live chat and all they did was reset and input some code but it didn't help, I'd rather find a way to fix this problem myself or from Networking professionals.
anyways here are the traces
Pinging [67.212.154.218] with 32 bytes ->
1 0ms 0ms 0ms 192.168.3.1
2 78ms 14ms 28ms 72.130.40.1
3 39ms 29ms 13ms 24.25.233.249
4 12ms 37ms 17ms 72.129.45.208
5 11ms 29ms 14ms 72.129.45.194
6 60ms 76ms 60ms 72.129.45.0
7 62ms 60ms 63ms 66.109.6.102
8 91ms 87ms 58ms 107.14.17.248
9 72ms 70ms 74ms 216.218.223.233
10 85ms 91ms 86ms 72.52.92.37
11 99ms 116ms 118ms 216.66.77.138
12 118ms 113ms 131ms 66.128.147.0
13 105ms 99ms 105ms 66.128.147.34
14 103ms 109ms 100ms 67.212.154.188
15 105ms 133ms 102ms 67.212.154.218
Traceroute complete: Destination is 15 hops away