My Aux belt was playing up for ages, squeaking, rattling etc, until I replaced the air con compressor, AND then because I waited so long the alternator pulley failed (it fell off), the noise only stopped after this and the power steering pump was changed.
The tensioner should NOT jump about, if it does and the problem isn't resolved it will cause more problems. I had a new tensioner assembly fitted because I didn't believe it when I was told the air con was the problem. 6 months after that when the noise returned and I eventually changed the air con, the tensioner was completely shot, the sleeve that is supposed to be a tight fit just fell out; not being OEM didn't help!
The only reason why the belt jumps is because the tensioner is trying to compensate for the belt being loose / tight caused by friction somewhere in the aux belt train. Try loading up the alternator (main beam / heater / rear window etc) to see if it gets worse.
You have my sympathy, my problem drove me mental.