Dear Prashant,

I guess you must have pulled out quite a few hairs off your head. OK no reason to go bald !

The way you have described, it does appear to be a EMI related fault and a flying one at that.

To start with some guidelines - I am sure other members will come in with thier own views :

1. Kindly recheck your grounding plan - particlarly when you have all possible kind of voltages roaming around. And what is a tree grounding - are you referring to Star grounding ? Do you have a separate earth or only the Neutral ? If its only the neutral, then you ARE in trouble.

2. Steppers can cause the most severe voltage spikes and need to be handled properly. I am sure the coils have high speed diodes like ( BYV27 ) as supressors. The simple IN4007 would not be adequate. And I dont know what drivers you use - if these are home brewn instaed of dedicated ICs, then that is another area of concern.

3. I thought you mentioned that you have a servo controlled stabilizer for primary regulation. IMHO it is at best a nuisance - you are that way well off with switchers for 12V, 5V etc. They will handle the line and load regulation beautifully.

A little bit more feedback on the above can help proceed further.

Raghu