You should pull the plugs and make sure they are the colder HT0 not the stock motorcrafts. the other thing would be to check the color of the injectors they should be BLUE at least. those are generally the cobra injectors so it makes sure you have pleanty of fuel.
Belt slip could be the issue also just a loose boost line if it is not electrical. the saleens use a sender for that stuff so it could be bad.
you should have a shop check it out.