There are several external reasons a spike could happen. Since you are talking about 1 Gbps I will guess you are using a STP (Shielded Twisted Pair) Cable, maybe Cat 5e:
- Water or water source near cable
- Close to power cables (Electric cables or electric wire in the building)
- Cable close to what I call noise makers, which are any device that creates electric magnetic fields (Like the cable going around a CRT monitor, Fluorescent lights or a motor for a printer for example)
- The RJ45 (8C8P) connector was incorrectly made
- The cable was stepped on, twisted beyond the 90 degree angle or near a heat source.
- Cable is not STP but instead UTP (Not grounded)
- Computer connected to the LAN is not grounded correctly (Hug the computer with love, if you feel a electric spike almost killing you then it is not grounded). This is known as static discharge. It can also happen by touching the cable which would be an insulation problem with the cable.
- Connecting Devices (Router, Switch) took a bit more to process a request.
And even with all of that, the trouble could be in the activities done in the network (Apps or processes that are running on the computers) or simple packages that are sent at the same time to the same destination, creating small lag while they get returned.
For what you mention after 100 ping requests I would actually say you have a very good connection. Doing that to google.com or even to a computer exactly 2 meters aways gives me worse results. I just did it with another computer right now and got 8 packets that took 1400 ms. So all in all, do not worry about the spikes unless you start noticing the lag increasing over all packets.