exhaust problem?

I have a 97 pont. transport,3400 engine. After not being able to fix the intermitent problem it has had for some time, the ses light is on and the code is PO420. The back pressure was checked on the converter and was found to be 3.5?. does this mean the converter is restricted? Could this be my driveability problem,(not taking fuel like timing is way off,among other things.)
converter is very expensive but if it will fix it I won't mind. please respond.
Thanks

This certainly could be your issue with drivability problems especially given the fact that you have a cat efficiency code. Actually cats are not that expensive anymore and you should be able to replace it for less thatn $100 in parts. Check on-line for a cat that will fit your car and then get someone to remove the old one and weld the new one in place. Most exhaust shops will do this for you. Good Luck!

I would occasionally get the same code last year. I got fed up with it so I just went ahead and replaced mine. I used a aftermarket converter (http://www.partsamerica.com/ProductDetail.aspx?mfrcode=MAR&mfrpartnumber=38763&parttype=145&ptset=A) available from Autozone or Parts America (Advanced Auto). I paid the $90 or so for it and took it to a local exhaust shop to weld it on. Took them a whole 10 minutes to replace. Costed about $50 which, if you think about it probably was highway robbery, but it's a much better job then I could have done with clamps and a heck of a lot quicker.

Haven't had a exhaust related code set since.