It will flicker if you're not revving fast above about 700-800rpm depeding on alternator or dynamo. My new one needs me to give it a good >1000rpm rev then it will charge at 600rpm for a while before starting to flicker.
If it is coming on when you are doing anything other than a slow tickover/crawling in a high gear its time to check:
-fan belt (state, tension, cleanliness)
-connections between battery, solenoid and alternator
-connection from alternator to light
-short between alternator light wire and chassis/engine
Do all that before suspecting the alternator. Twice I've replaced alternators (different vehicles) and its turned out to be crap wiring. The worst one I carried a very heavy alternator 10 miles round a Hungarian town in 40 degree heat trying about 20 different shops until I found a very nice bloke who did a re/brush, re-wind and new bearings for about £20. Put it back in and it still didn't work...then I saw that the charge light wire had worn through!