Is it crankcase pressure that causes it to pop-up? If so, check crankcase ventilation system/pcv. It might be possible that you have so much crankcase pressure, that ventilation system can't handle it, then the pressure gets out anyway it can.
On a warm engine, at idle, if you remove oil fill cap, is there smoke coming out?
Answers & Comments
Is it crankcase pressure that causes it to pop-up? If so, check crankcase ventilation system/pcv. It might be possible that you have so much crankcase pressure, that ventilation system can't handle it, then the pressure gets out anyway it can.
On a warm engine, at idle, if you remove oil fill cap, is there smoke coming out?