Diags? - Can't hurt when you have an inexplicable phenom, either all the time or intermittently. Letting Diags test everything and then run overnight, repeatedly checking memory:

As to "base 640K":

DOS is a crude theft of the CP/M OS, slightly redone for a particular microprocessor - as usual with virtually every M/Soft "innovation": stolen from someone who really did know how to innovate, a place called Digital Research.

Without going into details: "640K" in those days was thought to be vastly more memory than anyone would ever need! It became necessary to do lots of clever coding things to address memory beyond 1 MB and then ---> onwards.

IF.. there's a problem in memory within that first 640 Kilobytes (of your 256 or whatever Megabytes of total RAM) - usually the so-called POST (Power On Self Test) will stop everything, with beeps or maybe even a "bad memory" display - depending on exact address of first bad location, regarding the video display.

This would happen while still in the "black screen = DOS" first stage you always pass through before 'Doze loads all its toys.

Anyway.. running Diagnostics may reveal that the new memory IS bad (too) or improperly 'seated' or that the motherboard has suffered "infant mortality failure" etc. Electronics can fail any old time.. no matter what probability says! There's no reason Not to run Diags, so - why not run them?

PS: What I meant about "total memory detected by Windoze" was:

MyComputer/ControlPanel/System [Performance Tab] should show (at least in the 9.x series, dunno re Ex-Pee): what Doze thinks is the total "System RAM". almost always it will agree with POST results -- except in the case I described. Not your problem, here - apparently. Doesn't mean you don't have some subtle memory problem though; just less likely to be It.