Hi Ann. I'm not at all familiar with Protronix; I DO know, however, that jumpy readings sometimes occur with aged light sources. With transmission, typically a lamp filament is imaged into the reading area; if the filament is deformed and/or misaligned it doesn't always get the same light energy into the reading area.(posted 8187 days ago)In short, if you have a spare, I suggest replacing the lamp first. It's kind of typical for a spare lamp to be stored somewhere with or inside of a new densitometer, so hopefully you have one. Hope this helps.