Hi,
as Melanie said, you may have a weak power supply.
On the other hand, it is a good idea to check u r reset circuitry whether it brings the MCLR pin to logic high. Check this at the moment u r circuit does not start. Also at this moment, try to trace your clock. Does your crystal oscillates?

Perhaps a capacitor on the RESET network (if you use one) has leakage current to ground. Just an assumption.

Anyway, I always enjoy and read the answers given by Melanie
Regards
Kypros