Although it is possible that the OP's problem is not local, IMHO it is very improbable. With an upstream cap of only 128 kbps, there is little margin for error. The likely causes of delay are:
1. Unexpected traffic from the PC (malware, process running in background, filesharing, etc.)
2. Problem with cable modem or drop.
3. Oversubscription of local node.
Switching to another VoSP is very unlikely to help.
If the OP still believes otherwise, see if the test at »
myvoipspeed.visualware.com/ when using a server at or near the prospective provider's location gives better results than when testing to Phonepower.