intermitent rpm

I have a 2006 Liberty (it was a bank repo... I bid on and won last year...)

It rattles, rumbles, loses rpm, unresponsive gas then over accelerates, stalls at slow speeds, hesitates, and has lost all rpm to complete stall at 65 MPH.

When it is running well .... (LOL)it runs great!

When it is about to stall, putting it in neutral then restarting it, seems to reset it...

Had it at a mechanic for.. 2 wks.. 200.00...., they were stymied and changed the o2 sensors, drilled holes in the catallic converter it was mildly better but still hesitated.

That mechanic said it may be under warranty (for the catlytic converter) as it has 57,000 some miles on it (no 60,0000... yet) and said take it to the dealer. I am not sure about a warranty....

Taking it to the dealer in the morning.... so-

I am asking for advice....
is it the catalytic converter?
or something else possibly?
and does any one know what a catalytic converter should cost to replace?
(YUK)

Thanks!