I'm currently working on a maglock door security system, and am using these same chips to transmit data from the RFID reader to an Arduino about 200ft away. I as well have had all kinds of problems with these annoying, but awesome :), chips. I'm not sure I'm operating at 100% fool-proof yet, but here's a few things I've found out:
• There's a tradeoff between transfer rate and susceptibility to EMI. The MAX485 has a high transfer rate, but is very influenced by EMI. Try the MAX483. Same chip with slower transfer rate, but very little EMI.
• In some cases, such as with a long cable distance, I've found it to work better without any terminating resistor...don't ask me why. :)
• Make sure both DE and RE pins are connected to either ground or +5V. They aren't required to be connected depending on whether you're in transmit or receive mode, but I could swear I've had better results when neither of them are left floating.