I have also found the 18F4550 very sensitive on the pins when reading in simple high or lows, especially when using in motor applications.

If I run the same the code on say a 18F452 or 16F877 then not a problem, bu t as said the 18F4550 is very sensitive along with a few other quirks.

To get around this problem I doubled up of the sensing.

In other words, if I was was watching for a pin to go high within the code I would sample it, give it a short pause of say 100 us and and re-sample before breaking off to my routine. This seems to of ironed out the interference picked up by this chip.

Regards

Sean.