Ping problem on the computer?

I have had a new internet line (100k) for a couple of weeks.

So far everything is going great 109 download 49 Upload 10 ping, but sometimes not every day I have very severe ping problems. 20 pings in a row every 9ms-10ms and then one of the over 500ms then goes again about 20-25 pings 9ms-10ms and then again over 500ms and it goes on all the time. It stays that way for 1-2 hours and then everything is fine again and the ping does not exceed 20.

My computer is connected to the Fritzbox via LAN.

Is there a possibility in the Fritzbox to see which device is currently busy or can you see somewhere which pings times the Fritzbox gets directly?

My first guess was that it might be due to another device in the network, e.g. When a series is being watched on the television, but then it would have to occur every time you watch something e.g. On Netflix, but that was not the case.

Has someone probably already had similar problems or knows what else I can try or where I can look?

We recently had a very similar problem here, but I don't know if it really has the same cause.
For us it was due to the combination of a FritzBox that is no longer up-to-date with the latest firmware:

I don't know which server you used to measure the signal propagation times (ping) - we only measured the signal propagation times between the PC and Fritzbox internally - the times are of course correspondingly shorter, usually not more than 1ms, but the ping times then sped up for something more traffic deep into the 2-digit area:

This phenomenon occurred sometimes more, sometimes less and sometimes not at all.

In our case, the FritzBox 7490 with the current Fritz! OS version 7.21, which we operated via LAN1 on a 200 Mbps fiber optic connection.
Until the last Fritz! OS version was no problem at all for the 7490 but since the update to v7.21 these problems have occurred regularly and speed tests then showed not only higher ping times but also significantly lower bandwidths (down to 20 ~ 30 Mbps and once even under 20).

We finally copied the settings to a 7590 and replaced the 7490 with them and these problems disappeared again. The ping times are consistently low and speed tests again show bandwidths of 215 Mbps Down & almost 100 Up (as was normal with the 7490 even before Fritz! OS v7.21).

If you have a current Fritzbox, that shouldn't be the cause for you, but if it should be similar or even a 7490, it could also be exactly the same problem and then either a more recent Fritzbox or the 7490 would normally also be downgraded to the last firmware version will help.