Exact same feelings and experience as the OP.
Regardless of tweaksp apps or uptime my phone cannot do 4im chat clients (the main killer is msn, I've tried them all and they're all heavy on traffic), a text message and a phone call, it just grinds to a hault.

Using only the faceache plugin it seems to be ok.

I think its just the result of having a pc in your pocket, there's nothing to say allocate more memory and prioritize the processing power for, say, the phone app.