The stock upper swivel on a Hobie-18 is a pretty simple affair - no bearings or the like. If cleaning it doesn't help it may be time for a new one. The furler drum does have bearings, and Hobie sells a repair kit for refreshing those. The other thing here is check just how much tension you're putting on the rig. If you're really cranking the tension in you will have problems with the stock swivel, because again it is a simple piece. When furling on the water you'll find that furling/unfurling works much better downwind, as the tension is off the forestay.

The only time I ran the rig tight was in heavy air, and even then it wasn't super tight.

I never had any problems with the jib halyard block/sheave, would tend to agree that something is bent or worn there as well.