GEFANUC QuikPanel Set bits

I have three parallel inputs and a set coil output on one rung. The problem is the set coil will come on for no reason. None of the inputs have become true, But the coil will "set" on its own.
Using LD-PC is new to me however i am using a 90-30 I/O Rack, The machine is working fine except when this bit comes on, As it is an alarm bit so it halts everything. I have tried everything with no luck.
Get this: I can block one of the paralleld inputs with a "ALWAYS_OFF" and it will work fine, So you would think wiring problem huh? NOPE! it is a NC input so i Jumpered the input straight to VAC. Still acts the same !! Thinking input problem i even addressed the inputs to a different card, Same response!!
The set coil just does not like the input for whatever reason...
I have deleted all 5 rungs that are involved and re-written them, Still the same !!!
Anyone ever seen anything like this before ???

QuikPanel control using a 90-30 rack, Cimplicity ME software.
All inputs are 120VAC,

1) Your 'neutral' to the input module is intermittent. I assume you hard-wired just the 'hot' to the PLC. It's a N/C signal, so an open neutral will trip it.

2) Spikes on the neutral. Are there any big motors or SCR controlled heaters on the same feed?

3) AC noise getting in. To check, write more code with several inputs from the offending module, as well as others, to counters. Monitor to see if counters are incrementing. If so, check to see how grounding was done at terminals 3&4 of the PLC power supply. Bad grounding should screw up a lot of things, not just a couple of inputs. The neutral line is still the main suspect.

4) If you never find the problem or can't get rid of the noise, 'filter' it. Add a timer, set for 200 ms or so. Use Timer output to control the set/reset coil.

All Good Advice ! I wrote a few lines using the suspect input to set a "dummy" latch. Didn't latch those but the original set bit was still latching!
After much Adoo on the phone with GE We found that the PIF card in the quik panel was the culprut! Strange that it only showed up in this one bit. However after a quik card change of the PIF(comm)card the problem went away. SHEEEEWW!!!
Thanks for the advice and you were right on target about the common, but i had been through all of that.......MATT