Did you do anything to change your player power significantly at the times when you noticed the arena getting more or less difficult?

If anything my player power may have risen a little as I've been spending some silver upgrading gear, probably a few glyphs as well. 

I have otherwise not changed any gear and I have been using the same offense and defense lines. I've tried experimenting with swapping in a substitute here and there, I pulled some great champs in this last ancient boost, but keep going back to my core (Raglin, Arbiter, Turvold and Abbess).

Unfortunately, that's normal. I honestly don't think it's player power, since this kind of stuff has already happening since 4-7 months ago before plarium say they use player power for matchmaking 

What i believe happens is that there's a huge influx of players coming into the arena when you get the "easy" win, and then there's a huge removal of players when you tumble down the ranks.

The easy way to monitor this is the point requirements for Platinum, because Arena points system is a zero sum system the higher points required for platinum means there's more people active in arena, the lower the points, the less people.

What you do to your arena team doesn't matter, it's not your arena team power that is important, it's your total power as you can see in the top left when you're on the bastion screen. If you open a bunch of shards, your player power will increase whether you do anything with the champions you get or not. Likewise if you equip them or level them. You said you pulled some in the last ancient 2x, so presumably that increased your player power. Since you're in silver/bronze, that will influence your matchmaking to match you against players with similar power, which unfortunately means you can potentially get much harder opponents. For reference, at one point I was in mid S2, opened 42 ancient shards, and a week later I was struggling to maintain my position in B4 with no changes whatever made to my arena team.

If in silver then you are subject to the matchmaking system, how much is your total player power?

Player power is 1.2M

Should I dump some dead weight champs that I don't realy use and/or artificats I probably will never use?

I would dump any rares you definitely wont use - others may advise differently but it is almost certainly affecting your silver matchmaking

The community manager Oracle pointed out in another thread today, that

"The matchmaking algorithm also takes into account the previous performance of a player, not only his Tier."

https://plarium.com/forum/en/comment/topic_883754/  

So indeed there is a normal fluctuation, skyrocketing to much and you will meet other teams that did the same (and lose, because they are strong). I didn't know that before. Makes arena a pendulum, if you "win to often" you get harder opponents. It should be like that, but only because you are in a higher tier. Win often in Silver 4, go to Gold 1, meet harder opponents. But winning a couple of times in Silver 4, get stronger opponents allready here, losing against them, meeting weaker opponents - a pendulum inside the same tier. Why? Is there any logic-based reason for that?

I don't understand why they use all these strange mechanics at all. Player power, that includes faction war teams that never put a step into arena - just no!

They should do a kind of test and make the following matchmaking for a while: players of your tier, rest is totally random. What is the perfect way of matchmaking. The teams that are "to good" for their actual tier just climb up. Then Plarium should look if there come more or less complaining threads about arena.

So I think Iam experiencing the same. I was steadily moving from Bronze III to reach Silver I and then back to Bronze IV etc. I was 90k team power (Diabolist (240 speed), spirithost, warmaiden, Kael).

Then I dropped my Diabolist and put in a High Khatum (237 speed) and speed tuned my team way better. When my HK goes off all my champions go. So with the aura my HK is faster. But since I also replace some of the old gear (all L16gear) with lower level gear but higher speed my team power dropped to 60-70k. But when I went first I won.

But 1 day later I think the average team I face has 2 legendaries and teampower of 110k and higher and mostly defence/health/revive comps. As to earlier all teams I was facing where speed teams but better tuned. So I would kill all those early comps I was facing with my Diabolist but I get matched to way stronger and stirdier teams it seems.

BUT I also must say I think my Account power is also grown due to all the events iam taking part in and have been adding like 10 epics to my account aswell as decent rares and fusion champs. Have been dumping all the useless rares obviously but this is not enough.