How to monitor internet connection for interruptions - for Mac OS X

If you use the following:

ping -A -i 10 --apple-time 10.20.30.40 > monitor.txt

It will run continuously until stopped and ping every 10 seconds to 10.20.30.40 (change for your address)

The --apple-time means that it will log the time of each ping so you can see failures. Like so:

11:33:10.793801 64 bytes from 10.20.30.40: icmp_seq=0 ttl=58 time=27.744 ms
11:33:11.780250 64 bytes from 10.20.30.40: icmp_seq=1 ttl=58 time=9.757 ms
11:33:12.781136 64 bytes from 10.20.30.40: icmp_seq=2 ttl=58 time=10.150 ms
11:33:13.782932 64 bytes from 10.20.30.40: icmp_seq=3 ttl=58 time=11.779 ms
11:33:14.785446 64 bytes from 10.20.30.40: icmp_seq=4 ttl=58 time=11.254 ms

Applications->Utilities->Console.app

Take a look in the logs there as a start.

You could also look into ntop or Little Snitch.

How about ping running in Terminal? Just find a server that's on and responds to pings. While it doesn't show the exact times, it gives you some kind of proof that something's wrong.

Edited to add: I've used it myself a while ago for a similar. While they maintained that my (aging, to be honest) wireless access point might be at fault, since they didn't find anything, "I have ping timeouts at least once every hour" helped in getting the engineer to check on stuff.