A stronger external pull-up may help, but it's hard to say without knowing more about your
hardware setup, PIC type, code, etc..

-> This is a big project actually.
The main board features a PIC16F690. On each porta pins A0 to A4 I connected a TSOP4840 IR sensor demodulator at 40KHz with its 10UF capa and 100 ohms resistor (almost what's recomended in the DS).
When a PWM signal at 40KHz arrives on the sensors, the DATA lead of the TSOP pulls the PIC pin to GND so the PIC can receive the incming signal and decode the 16 bits.
I was just wondering if the internal WPU of the pic are good enough to prevent a false triggering... I didn't find the value of the internal WPU in the PIC DS.