PDP-8/L: multiple failed 7440's date code 7005

I have been busy debugging my PDP-8/L that last worked when stored in the barn around 1995. It is slowly coming back to life - still having problems with the core memory. I have noted one strange and recurrent problem. So far I have changed nine bad 7440's on a variety of modules, mostly core drivers (G221, G228) which have had their outputs stuck, usually high. All of them were labeled "DEC7440" with a date code of 7005! Currently lower page locations won't write the lower bits 5-11 (or at least return zeroes). The only common denominator for data bits 5-11 are at slot A09, where the MEM ENABLE 5-11 and MA ENABLE 5-11 lines are driven by, you guessed it, M617's featuring 7440's. Haven't verified this yet but it's a safe bet where at least some of the problem is! Has anyone else experienced this problem? Seems like the chip manufacturer must have had a few specks of dust in the fab room that day in Apr. 1970... -Charles