1995 940 L TD stalling problem

[marimba]

I have just bought a 940 L TD Estate, petrol engine, manual, first registered in 1995 into which the previous owner fitted a new petrol pump in August this year and I was told that thereafter the petrol guage stopped working and now shows no reading. The car seems to drive well on my half hour journey to work but is liable to stall and then not restart on my return journey home. I have replaced the relay with a new part and replaced the pump with a second-hand part but it still stalls. I have left the car running on tickover for about an hour, then when you rev the engine, it stalls and won't re-start until I leave it for an hour or so (I have not tested exactly how long I have to leave it!). When it is running the first I know that it has stalled is the lambda light on the dash coming on.
I thought that the problem was electrical and imagined that the pump was stopping and was going to run a separate earth directly to it, but it seems to me now that there may be a carburettor problem. Have you any ideas please?