It’s normal for the bike to jump forward when it has sat for a while. The EX500 / GPZ500S has a wet clutch; the same oil that lubes the engine also flows in/around the clutch. If the bike sits for a while, the oil will be forced out from between the clutch plates; the bike jerks when you shift from neutral into first because there is not enough oil between the plates for them to slip.

You can avoid the bike jerking by pulling in the clutch after you start the bike, and then holding in the clutch while the engine warms up. If the oil is cold, pumping the clutch a few times isn’t enough. After the bike has been fully broken in (after say at least 5000 miles) you could switch to a synthetic oil; that will lessen the tendency of the bike to jump forward.