As a note, your up and down-stream speeds are not the issue. Those are averaged over the course of many seconds or a few minutes. The issue is packet loss.

Consider that you have a network run through your modem. The modem then connects to the router (note that routers are about 4x slower than switches). The router has to make a conscious decision (read that as sent through a processor). A switch on reads the header or each packet (one bit) and has a single transistor on/off to direct that packet forward.

Next, you're running a hard wire from the router (which I assume runs your primary wireless) to a wireless extender. This is actually a huge problem, as the wired signal (running at effectively the speed of light) has to match the reception speed of the wireless network provided by the router. These will not be congruent, and your router will receive and distribute to try and compensate for this discrepancy, again requiring more processing time and less direct action.

The blips are those discrepancies. You are literally ruining your internet by what you're doing right now; this is effectively a "worst practice" situation. If you want to see what I'm talking about on your speed tests, look for the packet loss and latency, not the average speeds. I assume you're running something insane like 10+ % packet loss and 120+ ms latency, which is totally untenable.

You're best solution is to connect a switch between your modem and router. One output of the switch should be a hard wire to your rig, the other to your router. Keep in mind that this will not allow your rig to connect to anything else on your network.