Pete,

It looks like you can get a new solenoid from Rockauto in the $30 range:

http://www.rockauto.com/en/catalog/chevrolet,1981,chevette,1.6l+98cid+l4,1040106,emission,mixture+control+solenoid,5156

With the car getting that low of mileage it could be you have also have a fouled O2 sensor. You may also have another problem that disconnecting the solenoid helped mask. Is the check engine light on? Is it giving you any codes?