My son was taking Ritalin at 5 years old and everytime the medication started wearing off around 12:00 at school he would also go into rages, throw chairs and books around. We tried having them administer the dose at school by 11:30, but whenever they were late, this would happen. Same thing happened at home when his second dose wore off. As soon as we switched him to the slow release the problem stopped completely. It was all because of the come down from his meds.
We are also looking into possible autism (he is now 10) as there are still several meltdowns at school, just not violent or angry anymore. We suspect the cause for a lot of the meltdowns is sensory stuff. I think Penny mentioned this on her comment. I hope you figure it all out. It is so challenging. We are still waiting for further testing.