I'm playing a 10th lvl cleric. I attempted to cast an 8th lvl Power Word Stun spell from a scroll that the DM gave me. When I attempted to cast it the DM said it does not automatically succeed because I can only cast 5th level cleric spells. I don't see anything in the DMG or PH that suggests scroll spells are less successful when cast by lower level spell casters. The DM gave me a 30% chance of success and he gave me 3 tries (one attempt per round). Is this pretty common practice among the DM's out there??
It doesn't automatically succeed but the way your DM handled it was also incorrect with regards to the rules.
From the Spell Scroll's description on page 200 of the DMG:
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + spell's level. On a failed check, the spell disappears from the scroll with no other effect.