Staffs can suffer from a neurological condition which is called L2-HGA and one of the symptoms is 'tremors'. There is a DNA test which breeders can now use to test dogs before mating them, to ensure they will not produce affected dogs.
Ask you vet his opinion, and if he thinks it could be this then he can take a blood sample to be sent away for DNA testing - details below:
http://www.staffords.co.uk/downloads/l-2-hga.p df
No-one should ever buy a Staff without the parents both having been tested for this terrible condition, and no-one should breed from their Staffs without testing first. If you do, then this will carry on being a problem and many of the dogs will end up being put down. All responsible breeders will test, so only buy from tested parents. A cheap dog is not always the best buy.