Hmm. I've never experienced this at all: the input buffer amps on my 1624 are very stable, and levels have remained consistent at all times. I'd be inclined to look elsewhere in the signal chain for this one: either a gain stage with intermittent problems, or a cable with intermittent problems.
I've had many, many problems with mic pres on inexpensive gear that have very poorly-behaved gain trim pots. Matter of fact, that's why I usually don't make small gain adjustments with the trims: I generally go all the way back to 0 and then come up on the new setting from there, even if I only need 3dB more gain. This allows the wiper in the pot to sweep any cruft off the track, and reduces the likelihood of intermittent obnoxiousness. It's a habit I got into with a very old, abused, and noisy Studiomaster board back during the dark ages... This would be my first bet. Even as we speak,
my Alesis Studio 32 exhibits this problem on the channel 7 input. Drives me freakin' _nuts_.
The second bet would be a balanced interconnect cable with an intermittent short on one side. This might easily cause a reduction in signal while the short persists, followed by an apparent gain in level of up to 6dB when it clears. Bruce V. just stumbled across this issue with his Masterlink in another thread.
I really doubt that it is the recorder. I'll bet a beer that it is something else, some trivial thing elsewhere in the signal path that will make you go "D'oooohhhh!" when you find it. This sort of thing is really the joy of live work, no?