Hmm, I haven't seen that before. There are only two things you do differently from what I do: 1) the brand of mineral spirits; 2) the brand of oil-based paint.
I've used Winsor and Newton spirits with artists oil paint over Tamiya acrylic before without problems. Maybe there is something in the Testor's enamels that is causing a strange reaction. Personally, I've never tried weathering with enamel paints, I've always used artists oils (the kind that comes in a tube).
As for thinners, I have switched from mineral spirits to odorless mineral spirits, which are a bit weaker than the smelly kind. That may be a factor.
It could also be that you are using too much thinner to streak your oil paint. Try not to flood the model with thinner. I used a brush that is just ever-so-slightly damp with the thinner. I dip the brush in the thinner, then dab the brush onto a paper towel to remove most of the thinner. Then I use that barely damp brush to streak the oil paint.
Hope that helps.