530sei e34 m30 running problem

i recently bought a 1989 530 from down south drove it back home approx 200 miles , 3/4 the way back started to overheat , stopped car and checked water which was obviously low , a new radiator had been fitted
very recently so we thought that perhaps it had air in system and was`nt bled properly , so off we set got home with no further problems , on driving the car around locally the next week , discovered that temp gauge was`nt heating up just going onto the end of the cold blue marker , i then decided to take the thermostat housing off and replace the thermostat , again thinking that it was probably stuck open , you no what im going to say next there wasn't one fitted , so i replaced it and bled the system , took it for a run and then it was overheating again , ive read an article which tells you to bled them over and over and over which i have now done , the car now still only registers on the end of the blue marker , but if i get in traffic it will go up to the half way mark which ive always thought on any car is correct , my problem is its still running lumpy , on checking the spark plugs ive discovered that out of the six plugs , 5 are what i would call perfect colouring with one , number 1 plug is black not oiled black but sooty black , can anyone shed any light on what they think might be the problem , another thing ive forgot to mention is at slow speed car does`nt seem to want to pick up ie when you put you're foot down on accelerator peddle it hesitates , once you get it up to 60 or 70 shes fine , cheers wayne.
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