Changing from the OE HD electrode to the standard electrode will NOT cause the idle issues you are reporting. Both should be gapped the same. The difference is that the HD version has a wider center and ground electrode. You have another problem!

Check that the throttle position switch on the end of the thottle valve shaft is sending the idle signal. As they get old the contacts can misbehave and cause idle problems. If the ECU does not get the idle signal from the TPS it will not go into idle control mode to maintain idle speed/stability.

Duke