What is messing up my system clock?

Ubuntu 14.04 desktop, 64-bit

I came in to work this morning and noticed that the clock on my system was off by approximately 3 minutes, as compared to the clock on my phone. That seems odd, since my system is on 24/7 and should be keeping the time synced with ntp.ubuntu.com, right? And I've never noticed this problem before.

In the system configuration, the "Time & Date" settings are set to set the time automatically from the internet.

So I started digging...

First I used ntpdate to update the time. This worked, but a few minutes later the time was incorrect again - by approximate -217 seconds.

Next, I installed ntpd, hoping that continuous updating would solve the problem. I reset the date using ntpdate and started the ntpd service. No luck - a few minutes later, the time was off, again by approximately -217 seconds.

The offset of -217 seconds seems pretty consistent - ie. it doesn't appear to drift once it hits that difference. This suggest to me that there's something on my system resetting the time periodically, and because the change is too large, ntpd will not adjust it.

After some more experimenting, I have determined that whatever is resetting my clock is doing it approximately every 5 minutes. I've looking in my crontab and the crontab for root, as well as the system-wide cron files.

A couple of additional details:

1. I have joined by system to a Windows domain using Likewise Open.

2. There was a major change to our network over the weekend - we switched from one connection to a different (faster) connection. However, I appear to be the only one experiencing this issue (amongst about 30 Windows PCs, Ubuntu PCs, and Macs).

So, a couple of questions:

• What could be doing this?
• If a scheduled job is not in my cron files, where could it be?