Thanks for your replies.
I have a A/F ratio display in my car, with this I've discovered that the hesitation is due to a lean mixture. It cannot be weak spark, because the engine hesitates when you press the accelerator (when its hot) but doesn't lose power at high revs like you would experience from a weak spark.
My current theory is that both issues are caused by improper operation of the idle valve. I believe that in the scenarios wherein the engine stalls, its because the idle valve doesn't cut in fast enough to catch the revs before the motor dies. My reasoning for this is that as the motor is heating up but not at operating temperature, when you press the clutch in and let it idle, the revs often drop down very low (maybe around 500rpm) before quickly coming back up to a steady idle. I can imagine this is because the idle valve cuts in just in time, but for some reason as the engine gets hotter the valve cuts in later and later.
The hesitation could similarly be caused by the valve staying open momentarily once the throttle is opened, letting a very lean mixture into the engine (confirmed by the A/F display).
So my question is, why?? What sensors tell the valve when to open and shut?
LoneWolf33, wouldn't removing that vacuum hose mean the injectors are always receiving ma fuel pressure, giving the engine a very rich mixture at idle and low rpm? The A/F display shows that my mixtures are generally good across the RPM range.