Web Master:
Hmm, seems like a long and determined struggle there John. My guess is curl grubs which tend to be introduced by compost - the variation in timings is the clue here. Have a forage around in the soil and see if you come up with any plump, creamy coloured grubs - if so then you'll need to clear out the whole bed - just sieve it and leave the little critters on the path so the birds can eat them.
As to what to plant instead - I'd suggest Westringia fruticosa - same colour but a bit taller, though you can keep them trimmed lower. Natives and very tough, with pretty mauve flowers.