I mean, I deleted my current FP, program in a new one, go into the VNAV page to enter a new altitude, and as soon as I enter the alt and hit enter, FS crashes.
It's only happened once so far, the first time after a flight from HKY to JWN, I re-programed the FMC to ORK, and I got the VNAV page crash. The second time was after a flight from JWN to APF. I programmed it to go to MEI, and I got another crash after entering my cruise alt.
Hmmm try as I might, so far I've been unable to re-create this behavior you are experiencing.
Can you provide more details ?
Example the first flight plan, was it created from scratch ? or loaded from a file ? If it was a file , was it a .rte file, or a .pln file, if so do you still have this file ?
If it was created from scratch (ie via the new fpl LSK), can you list the sequence of steps you used to load both the first and the second flight plan.
Also during the first flight, did you make any route changes ?, insert or delete any waypoints, insetr any Sid/Stars's anything like that ?