why does naomi inject herself with the syringe throughout the game, did she want to die? what is the cause of snakes random seizures? are there two types of syringes, one that boosts nanos and one that supresses them?

The nanomachines inside Naomi and Snake sometimes play up due to various reasons such as Their bodies dying or SOP rejecting the nanomachines.

The syringes inject nanomachines which suppress those effects and limit what the main body of nanomachines inside a persons body can do. ( As such they limit the speed Vamp and Naomi heal ).

Also while we arn't told so i believe the syringes help Snake and Liquid to control their bodies -- after they did inject themselves with ALOT of nanomachines during that last fight ...