Thanks for the suggestion FireFish, but as it turned out, this was not really the issue, as there were no monsters directly touching on the on map lifts directly. :)
So, long story short, the problem is fixed. (Fixed, tho not solved, for reasons we shall soon see.)
The switches that were crashing the level had a linedef tag of 5. Now this sector tag was linked to three sectors. Two lifts next to the switches, and one off-map monster closet. When you hit one of the switches, it would lower the two lifts, and the third sector around an off-map archvile, allowing it to teleport onto the level. (The archvile would not be woken up until much later in the level, so you would not trigger it until a return journey past those lifts much later in the level). Removing the archvile fixed the problem, the level would no longer crash when you hit those switches, so NT and everyone, you were correct. I tried to widen the area around the archvile, raising ceiling and floor heights and moving that sector around to different points on the level, but no matter what I did, as long as that archvile was even in existence, the level would crash, irrespective of how close or far it was from associated linedefs.
So archvile deleted, problem fixed. But here is the interesting thing. This archvile never used to crash the level, but at some point, as I continued to make (relatively minor) modifications to the level it suddenly did start crashing. So the deeper issue is that the archvile, along with other unspecified changes, would cause the level to crash, as in its original state, the archvile did not crash the level when the relevant switch was hit.
So in the end I took the simplest solution. Arch BALEETED! Problem given a most fFIXACIOUS solution! So, problem fixed but the deeper issue not really solved. Much appreciation for you guys pointing me in the right direction. Soz if this post is rambly.