I recently traded JDW my spare working G3 300 for his G4 500. So, I have the same upgrade. I will warn you, it runs VERY hot.

Intermittent boot issue:

Check PRAM. Mine is dead as a doornail, with no issues (except losing time) and I assume they work at full power. Yours may be in a middle state. Try removing it.

Not booting from CD:

Mine also came (back in June) with a dead DVD drive. I purchased a CD drive from Tonyzzz on eBay and it has worked excellently since.