misfiring issues need adivce

My 86 cargo conversion has been acting up. the van has 140,000 miles on it and is in considerably good shape both inside and out. it all started with cylinder #3 going dead so i did a tune on it (wires, plugs, cap, and rotor) and it ran great for about a week before the cylinder went dead again. Since then i did some research and was convinced to replace the fuel injectors assuming that one was clogged (the van did sit for awhile before i got it). After having my mechanic replace the injectors and injector plugs it ran worse then before, misfiring, shaking, not able to stay running. we have since tried replacing most of the components of the electrical system (ecu, ignitor, distributor, and some sort of injector control module) with no luck. possibly vacuum issue..?

At this point my mechanic (who is familiar with these vans) is stumped and about to be over it. this is my second van and i have only had it for about 6 months and really don't want to give up on it. i am not a seasoned mechanic by any means but have personally put a lot of work into it prior to these issues (alternator, new hoses, belts, battery, water pump, fuel filter, front main seal, tires, front brakes/rotors, and shocks) and it ran amazing. if anyone has any ideas or suggestions it would be greatly appreciated.

- thanks

In order to replace injectors the upper half of the intake had to be removed. If replacing injectors made it run worse, then I'd have to say there was a mistake made (assuming the "new" injectors are good). It might be something as simple as a hose knocked loose from the air valve or a disconnected vacuum hose. It could also be a damaged gasket between the manifold halves. Sometimes wires or other objects can fall down during assembly and get pinched in gasket surfaces (and create vacuum leaks). Regardless, I think you'll need to scrutinize this area.........perhaps pull it back apart & verify the job was done correctly. Tim

PS: Make sure the air valve hose is connected to the tee at the bottom of the throttle body intake tube. This is the bigger one (about 1/2" diameter) on the back. It's very easy to miss this and hard to see if it's not connected. You'll need to feel back there to be sure.