Odd ... If CDS meters "drifted" with time, I would imagine another system would be required for "Flash" meters. After all, the maximum flash duration I get from my DynaLites is 1/450th of a second. The Gossen UltraPro I use hasn't shown **any** drifting that I can see.
There are minor - very minor - variations in readings - to be expected - those variations are "precision" errors, and SOME error is unavoidable in all measuring systems - not only photometer - "Light meters".