The company needs to get someone come in and take a look at the issue. Run a packet sniffer on the network, find out if the routers/switches etc are running optimally.. One piece of bad hardware could mess it up, specially if it's right where the network leaves the company and goes to the ISP.
I would have a professional investigate the issue as it could be a lot more complex then it seems, Could just need to reboot the network completely.