1991 Dodge Dynasty 1991 dynasty engine computer

I have a 1991 dodge dynasty 3.3l v6,automatic trans,cruise control and no antitheft.Last July,2007...I went out to go to work,and,the car just started and died,over and over,would start and shut right down.towed to mechanic,who proceeded to troubleshoot:tac valve asmy,cam-map-crank-sensors decided it must be the ecu.To save me money got a used ecu,1991 new yorker,no.4686430,(I looked on casing later).I'm sure he used number off original.Anyway,It lasted four months.I stopped at a store,came out and the same problem.Starts up and dies right down over and over.Of course I called the same mechanic,who did some diagnostics and said its probably the used computer.So,we purchased a remanufactured ecu,at a higher price,and then it didn't start or would start sometimes and not others.He has replaced many parts that you would think pertain,asd,speed,etc..They thought it could be a bad injector,then,the harness itself.This has been over a month now,from about Dec.10,2007 to today,Jan 21,2008.
Any help would be Highly Appreciated,and I read where the revised sbec-4686442 could help!Can I use this in place of the 4686430 ecu? I'm in dire straights!! Please help!! Dave