On a 96 I would think you either removed the intake manifold, in order to get to the glow plugs conveniently, or simply struggled working in the tight quarters. I recently discovered on a friends 606 powered vehicle, it had glow plugs replaced last year, burned one of the plugs in about 8 months. During replacement I discovered the plug that was burned was loose in the head. I do not recall him having an idle complaint but it did cause the plug to fail within a year.

Only other thought, did you ream the glow plug bores before installing the new ones?