1986 300 SEL engine trouble, please help

Hi there,
I own a 1986 300 SEL 6 door limousine that has recently started behaving in a peculiar way. When I start the car the idle is very rough and when I put
it in drive it feels very sluggish. I can floor it but there is only very slow acceleration. The car feels like it drives only on 5 cilinders. But when the engine reaches about 2000 RPM the 6th cilinder kicks in and the car behaves like it should. Also the economy meter shows hightened fuel use until the 6th cilinder kicks in. When driving it is very slow on the road and then around 2000 rpm it suddenly drives normal and I can attain higher speeds normally. When I slow down it starts all over again.
Could it have anything to do with the spark plugs or the spark plug cable?
TIA
Marty
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Sounds exactly like spark plug itself or the cable... but most likely cable. Check all the cable for resistance... it should be no more than 1350 ohms.
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