We have really thick grass here in Oregon and I have the same mower you do and it squeals if I try to mow too fast. Sometimes I have to go ridiculously slow.
One problem I did have with my mower however is that the bolt holding the idler pulley bent over time to where the pully was actually rubbing on the deck. Before the pully had actually rubbed I couldnt tell there was anything wrong because everything felt solid and would pivot without any problems. Just after the pully started rubbing, the bolt broke. I think it performs somewhat better with a strait bolt in there. You might just take that bolt out and examine it.