How come the "Tx Rate" on my Airport card changes in my MacPro,4.1 on Yosemite?

I see no playable video that I can open at the link you posted.

what do I do to see your Video?

The changes from 30's to 40''s and back is normal and is likely due to interference from other networks since you show 5 other networks and other sources.

You are on a G network which is prone to interferences since many things like phones and microwaves are on the frequency (2.4 GHz).

Your value of 40-50 is lows but not unexpected on a G network.

I had similar rates with my G network. That improved to over a 100 on a 5 GHz N network and on A AC network I have 585 (2013 Mac Pro).

On my 2009 Mac Pro I have 243 with a 5 GHz N network

When you start, your Mac tries to broadcast at it full rated speed (54). As it encounters errors, it adaptively switches speeds down to a lower speed in an attempt to get a reliable connection. I expect if you continued to use this awful connection, its speed would go down even more.

• Your signal level of -82 is horrible, and only a few dB above the noise level at -89. You need at least a 20 point signal-to-noise difference for good reception and high speed.

• You are using the very busy 2.4GHz band, which has a lot of potential interference. There are only THREE clear channels there, and you have four other competing networks already.

• You are using 802.11g mode, which is antique, and has a top speed of 54Mbits/sec. 802.11n mode in the same conditions can go over 100Mbits/sec.

You are either too far from the Base Station, or your internal antenna is disconnected. Your Base Station lacks modern capabilities such as 802.11n or faster, and does not appear to have the additional 5GHz band (or you would automatically be using that for better performance).

You need to address these problems directly:

• you appear to be too far away from the Base Station. Most users can move the Base Station to a better location if they think creatively, and extend the cable or Phone line to another part of the house and find a new electric outlet there.

• Your Base Station lacks many features of a modern Base Station, and it is about time to get a new(er) one.

This is what mine looks like in Mavericks with the new "tall" Apple Base Station:

I am using 802.11n on a Mac Pro 4,1 (that is as fast as mine can go, a limitation of the Wi-Fi on this Mac).

lllaass has a newer Mac that can take advantage of 802.11ac.

But this gives you an idea where current Router technology COULD be taking you.