2002 3.4L throttle bog??????

Hi.

I have an intermittent part throttle driveability problem with my 2002 Impala 3.4L w/93K on it. It feels like an old fashioned carburetor “bog” from back in the day. (and back when I knew how to fix such things!) It starts and idles perfectly, but at least half the time under part throttle acceleration, it stumbles and hesitates. If I leave my foot in the same position it accelerates very slowly then, all of a sudden “clears its throat” and accelerates smoothly right up to the shift point. Only once out of over a hundred times did I get a check engine light, and the code translated to “EGR valve outside limits". We have replaced air and fuel filters, had the fuel system cleaning (Jiffy Lube), replaced the plugs, flushed the transaxle, checked fuel pressure at idle and under load and replaced the EGR valve. No dice. My mechanic has spent way too much time (most of it on his own time) quizzing the computer, and verifying that all the various sensors are operating normally. He’s a very good technician and, if he’s stumped I’m at the end of my rope. In reading other posts on this forum, it sounds a lot like the plugged catalyst symptoms, but that would happen all the time and this doesn’t. Does anyone have any ideas?

Thanks in advance…………

Well, it's fixed. Clogged catalyst.

I have to say it now drives the best it has since I bought it with 65K on it. This clogging must have already started at that point and just got worse over the next 30K.......