I can't see any 100nF-Cs round your PIC - is he resetting ? You can use a portpin to see it!
Maybe it will better the AD-Converter by filtering his AVDD via 56 Ohm and 100nF against ground, but this is not the reason for your problem.

The INTHIGH-routine starts the AD-conversion on every Interrupt, because the bit-check is faulty (or not complete).

For PIC18-uCs: Writing to a port (PORTA) is better done by writing to the LAT-register instead of a PORT-register.