UKPassats

My B3 1991 has irregular firing and if left standing the engine will cutout. If driving (short trips only!), the engine will start to 'die' so I have to dip the clutch and try to get the revs up and then release the clutch. If I get to slight inclines, it gets worse and the engine tries to die (and does sometimes) and I have to stop and try and get revs up to speed, but they soon drop off when I engage the gear/clutch again.

I recently changed the lambda sensor and water coolant sensor although that didn't have any immediate effect. Also I recently filled my tank, so could it be contaminated fuel is the problem? I am thinking of putting back the lambda sensor and water coolant sensor just in case they were faulty, but I am clutching at straws

Any help would be appreciated.