I would suggest to use an opamp as a unity gain buffer befor feeding the ADC of the PIC.

As Charles stated 350K is too high for the ADC to work reliable.

Also to be sure that the PIC is working OK, use a trimmer in the place of the phototransistor and check the results. If they are stable and seem expected then you can be sure that PIC is OK. Of course it would be a very good idea to average the samples. Have a look at Darrels averaging routine at his site:

http://www.pbpgroup.com/modules/wfse...hp?articleid=7

Ioannis