I doubt it is a bad SOV.
However , it is possible..
More likely it is a plugged egr pipe to the intake [common prop on 104 eng]
There is an article on it in the DIY section and the Search Archives.
An easy test is to apply vac to the egr [ hand pump/mighty mac/etc] while at idle. Eng should run rough w/vac---smooth out as vac is released.
A SOV test is to look for vac at the line to the SOV and the line from the SOV to the EGR.
Common prob in this area would be the plastic line feeding the SOV [ under front plastic eng. panel] becomes brittle and crack...
[ thus, no vac to SOV]