Intermittant battery/starting issues?

[alpenweiss2]

Over the last week or so we have had some starting issues with our 1999 1.9 tdi wagon.

I have suspected a dodgy battery however it has a Varta Silver F18 with 85Ah and 800CCA fitted and the battery is only just over 3 years old?

I have cleaned up the terminals and positive and negative leads and tightened up and smeared in vaseline.

Some time the car starts instantly at the turn of the key and other times the battery barely turns over the engine and the lights on the dash flicker?

Could it be a bad connection to the starter motor? I have been in touch with Battery Megastore and they say the battery is covered under a 5 year warranty. However we are now living in France so returning the old battery and getting a replacement will not be straightforward?

I am not sure it is the battery as sometimes the car starts easily and other times it won't?

I have checked the charge when engine is running with a multimeter and it is getting above 14 volts.

Any thoughts people?