I feel your pain, not easy getting these old dinosaurs to comply to a test they weren't designed to pass when they were originally built. Been down this road many a time! I have a couple of suggestions, these are what I always checked out before heading down to the smog station. Question : is this the same cat that was on the car when you smogged it 2 years ago? check for any exhaust leaks (head to cat), coolant t/stat ok, check EGR valve operation (disconnect hose, apply vacuum to hose going to valve at idle, listen for idle change, also look down at EGR valve, and see if valve stem is actually moving, if it's moving, but idle doesn't fluctuate, it's plugged up/ not flowing), check for vacuum leaks in intake (spray brake parts wash at different areas, and listen for hissing/ idle fluctuations), areas of concern are IAC hoses (splits usually right where hoses go over the hump of IAC valve), splits in intake accordian hose between MAF and TV ( take it off, stretch it out and look for splits either in the ridges, or valleys, check pre heat t/stat in lower area of front half of air cleaner housing (these are almost always bad, they need to be fully closed, with tension on the flapper, when they're open you're running exhaust heated air into the MAF, could cause some leaning out, and WILL KILL your MAF), check fuel pressure regulator (pull hose off at idle, check for idle change, and there shouldn't be any fuel leakage, check EGR amplifier/ hoses for leaks (split hoses/ loose vacuum elbows), check oxygen sensor for good swing in operation. Assuming that plugs, air filter, clean engine oil etc OK. If everything checks out OK, I think you're probably looking at either MAF or CAT. I'm sure you've probably done most of this already, if not all of it. Hope this might be helpful, and good luck.
PS : definitely take it in after a hot road test, and don't don't shut it off while you're waiting in line