Have you replaced the fuel filter? A clogged filter will starve the engine during acceleration. That can also happen at the pick up in the gas tank. Gummed up injectors can be a problem too (they're a PIA to replace). But I don't see that being your problem. With misfires on all cylinders occurring randomly, I don't see it being a problem with the plugs and wires, either. I'd look upstream and downstream of the cylinders for the root cause (intake and exhaust). I'd also check all of the emissions related parts (EGR and all the little vacuum hoses on the parts around it). The cat might be worth looking at too, as you mentioned.

And I agree with Everet - fix the heater core at the earliest opportunity.

1997 Sportage 4x4, auto-trans, Warn manual hubs, 4" UPYOURKIA front lift, TJ 106AA rear springs, 2-5/8" body lift, 31x10.50 treads, SmittyBilt SRC front and XRC rear bumper, swing-out tire mount, OBX LSD front diff, Track Finder rear locker, 5.38 R&Ps and... really crappy gas mileage! <img src="/forums/images/graemlins/shiner.gif" alt="" />