Thanks, here is what I did.

I can see there is no simple solution for this problem. I have tryed all combinations with three computers and cables. The results are:
1) all computer works fine as long as the cable is short than 20 feet.
2) none of the computer works as long as the calble is longer than 30 feet.
3) I made three cables that longer than 30 feet and none of them work.

The guy work in Fry's told me that I need to buy the connectors with gold on the tip. It is expensive! I would try nest: to buy a premade 100 feet cable with connectors at each end. If this one work, I will cut off the connector and put one on by myself. It it stops working, it will prove that it is I who has problems here. Thank you all very much. And keeping suggesting. I would love to hear from you.