Some suggestions...

1. I assume 33pF and not nF!!! Even 33pF seems a bit large for a 20MHz xtal, whilst 33nF should be enough to kill your Oscillator dead!!

2. 7v into a 7805 Regulator is really 'marginal'. What's the 5v rail like under the worst case load conditions?

3. PIC Restarting could be noise on the MCLR pin or the volts dropping to the Brown-Out setting (see point 2 above). The Brown-Out setting I believe is software adjustable, it may be set too high.

4. If you can... get a 18F2520 just to debug your board... it has two features... (a) you can run on internal oscillator (but not at 20MHz) and (b) you can run on internal MCLR. Those two features can help to identify if you have external Oscillator or MCLR problems.

5. Very very early 18F24x's and 18F25x's (I'm talking some three years back) had heaps of problems as they were very fussy as to noise. A bad board design could kill the whole project. Make sure you haven't got one of those.