Which internal clock are you referring to? Which PIC are you using?
If you're talking about the machine clock stopping when you remove the 6mhz external clock, yes, it will stop because the internal 24mhz clock is being driving by the external 6mhz clock.
If you're talking about the internal 8mhz clock which is seperate, then, no, it won't stop and you can make use of the clock fail interrupt and switchover bits.
Don't forget that any clock switching takes xx milliseconds to restart itself internally.
Or maybe I'm missing what you are doing here.
The PIC runs, does some stuff, stops and waits for an H-sync, then starts up again? Could you sit in a tight loop waiting for an H-sync on an interrupt pin, then do what has to be done?
Or again, maybe I'm missing the point entirely...
JDG