You are mixing your modes!! For vertical navigation you can either use LVL CHG, VNAV or V/S. Selection of any of these will knock out the previously selected mode. So if in VNAV PTH pushing LVL CHG will cancel VNAV. ALT INTV will remove the next constraint from the FMC. For example on a SID first waypoint has an altitude constraint of 4000', controller clears you to 6000' before you reach the waypoint. To stop the aircraft leveling off at 4000' in VNAV PTH you set 6000' in the MCP and press ALT INTV which clears the 4000' restriction and allows you to continue climbing. It's a complicated subject, if you want more detail I'd be happy to help. Andy