The pas pump would have let you know if it was failing, as an engine is loaded at one particular throttel opening as the revs drop the torqe produced increases, it is possible to place quite a large load on an idling engine before it stalls, also pas pump and oil pump problems in general tend to show up as a lack of pressure first, then they start to get noisey and then shake thenslves to bits. if it was seizing you would also hear a lot of belt scream at normal driving rpm