This is how analogue flickerfixers work.
Analogue circuits always drift due to temperature changes, so just let it warm up and then adjust the picture with a good set of flickerfixer test patterns.
Digital scandoublers/flickerfixers don't exhibit this problem, but you can't connect one externally - it needs signals from the chips.
I guess I should tune my A2000's flifi again, it's gone terribly out of sync.. Then again, I normally only use it for the early startup menu..
