i'd shoot a mag. leave it an hour. shoot another mag. if a massive drop, you have a leak.

there may be other things going on, but try the simple stuff first..

PS it also look a lot like a dodgy regulator (leaking through), but you said you haven't got one ?

the real throught process here, is is the sawtooth drop time related ? If time related, something will be leaking - either out of the gun, or through a reg/prechamber, or somesuch.
I can't see how the mag change per se could affect it - unless the gun is blowback. Other option is single load, and chrono every 3rd / 4th/5th shots to save boredom - the other shots can be target practice, plinking whatever. is it a smooth decay ?

I like a challange - much better than .177 vs .22 or TX vs HW