If you had a 1977 DT 400 you could easily fix that pulser because it´s not enclosed in plastic. The 1977 Dt 400 pulser looks like a miniature lighting or source coil. If everything is testing ok try cleaning YOUR stator connectors. I actually sand blasted mine to make sure it was completly clean but I made a mess. You must be carful not to sandblast too much or the connectors will be loose. I guess you could just crimp it a little more after.
Since your motorcycle ran fine with your friends stator It sounds like something is throwing your timing off when your stator is installed. I guess your pulser or source coil could be shorted BUT NOT OPEN and that could cause your timing to be all out of wack but the motorcycle could still idle. I mean electrical is weird and if your source coil or pulser coil has low resistance because of a short It could give a weak spark or even throw your timing off. A pulser with a short may still let the motorcycle run but the signal may be too weak going to the CDI box and cause all kinds of weird problems like weak spark and possibly timing being thrown off.