Sounds like your engine is set slightly lean. The extra g's you are pulling in a quick pull up starce the engine and you aren't rich enough for it to recover when going vertical. Hold your plane vertical on the ground for several seconds, maybe 10-15, to see if it leans out or keeps running. If it is max lean in a prolonged vertical, it probably won't stand the extra g's of a snap pull up. In this case, you'll have to run richer.