GM-GMC: 97 GMC sputters then dies, gmc safari, muffler repair

Question
I have a 97 GMC safari van with the 4.3liter engine. My wife was driving yesterday and it just stalled on her. Had it towed to a garage. They indicated the cat was plugged and wanted a rediculous amount of money. I was able to drive car home. I tried to get to muffler/repair shop and made it about 1/2 way. The van sputtered and stalled. I let it sit for a few minutes and got it started. Made it a little ways further and it died again. Waited a few minutes and finally got it to the shop. Every time it stalls there is a wafting gas smell. This shop check cat and said it was not plugged. They tested the engine and indicated that 3 of the 6 cylinders are "leaking". They indicated that they don't build any pressure. After the van sits they will build pressure then loose it again after it is running. There doesn't appear to be any smoke coming out of the exhaust when this happens. I have never heard of anything like this.

Answer
I am a little unclear on what exactly is happening.  Let me give you a little bit of back ground info.   Your engine needs three main things to run. 1.compression 2. ignition (spark) 3. fuel   I am unsure what is meant by losing pressure.  If half of the cylinders are not able to create adequate compression to support combustion, than yes, your engine would run very rough, if at all.  Now, the plugged exhaust, there are two ways to check for a plugged exhaust.  One is to install a back pressure gauge in an oxygen sensor port on the engine side of the convertor and monitor the back pressure.  The second is to simply unbolt the exhaust from the manifolds and quickly road test it.  The first method being the preferred method.  
My suggestion to you would be to find a reputable repair facility that you trust and have them diagnose why you have a rough run/stall condition.
Hope this helps