My 1990 Lumina has developed a stalling problem. It starts and runs fine, but then will shudder and stall when slowing and gearing down. It feels like it is lugging and in too high a gear, then stalls. It will restart, but stalls again as soon as put into gear. After a while it will start up and run normally, until it decides to do it again! We thought it was a vacuum leak - found one and fixed it but the problem continued. Then we disconnected a wire with a blue plug that goes into the right front of the engine. That stopped the stalling, but it won't go into passing gear now. Is that some electronic modulator that is faulty?