Here are some more tricks and tips that will help you improve the things.

1. Use good filtering and decoupling for supplying the micro. Here is what I do for my designs.



2. Have a solid ground plane on your PCB and use ground gaurds around all your high speed signals.

3. Eliminate ground loops. If any.

4. Check the current ratings of your micro IO pins it may be wise to use a buffer for optocoupler feeding.

5. Do a search on the forum this issue has been discussed several times.

6. Read the hardware FAQ section there are some good articles on noise reduction. by Erik Malund and Myself [ writen with the help of KAI KALSS ].

Regards,
Prahlad Purohit