Climb in VNAV

Hello PMDGFirst I'd like to thank you for such a great product as the 400X. I fly a few other good quality aircraft in FSX (no names) none to match the 747, the sounds are amazing. However, flying PANC to EGLL I was performing a step climb from FL330 to FL370 and was cleared by atc to FL340. I dialed 340 into the MCP and pushed for ALT INTV, THR REF LNAV VNAV SPD was annunciated. As I was about to capture FL340 I was given clearance to FL370 so I dialed 370 on the MCP by which time SPD LNAV VNAV PTH was displaying so I pushed again for ALT INTV checked the FMC and FL370 was showing as my CRZ FL, however and here's the problem, I continued to climb from 340 to 370 at 500 fpm with SPD LNAV VNAV PTH annunciated, no matter how often I pushed the ALT knob I continued climbing in VNAV PTH mode. Have I done something wrong or is this a bug? Any comments would be appreciated.Matthew.Sorry if this has already been dealt with :-)