I have had a court order since 1993 for support of one child $100 per week.

The non custodial parent did not pay a penny until my child was 16. The amount owed was over $60K and with interest well over $100K.

He finally was arrested on contempt and forced to pay. He has been paying for approx 2 years and now I am receiving reduced payments. I asked DOR why the payments are reduced and was told he could possibly have another order he needs to pay. I found out that was true.

My child was his first and there is no order for modification on my case. Can they just automatically reduce my payments without warning or any order?

Yes, they can. You can only squeeze so much blood from a stone. There are laws that limit the amount paid and apparently he has reached them.

You can return to court for a modification, but it will likely not work out in your favor since things are already being handled by the state.

Yes, they can. As pointed out you can try to go back to court but it won't work out in your favor because he is still paying what he can pay. There are limits on the percentage of someone's check can be taken and that limit has obviously been hit. Even though your child is older the other order has to be paid as well.