After 140000 miles I started to notice strange moments for my Roadmaster '96.LTD. .It feels like engine is missing .When it happened first I just thought I hit some small bump on the road but then after several months those moments appeared more often.They are very irregular.Sometimes more often but sometimes only a couple times every mile driven. Can only notice them when driving at the constant speed over 50 mph. And sometimes when stopping for red light with your foot on the brake. I replaced spark plug wires but that didn’t change anything.
Any ideas ?
Sorry about my clumsy english 
What about the spark plugs and the distributor cap and rotor?
Are you sure that it is an engine miss and not a transmission shudder?
When was the fuel filter last changed?
I replaced fuel filter just 2 months ago. Spark plugs are still in good condition with something less than 40000 miles on them. Optispark distributor set was replaced in 2008.And I just use this car for summer time traveling only. All the rest of the year car sleeps in a garage. So there isn’t too many miles on distributor.
I’m not really sure if this is engine or transmission but when I sometimes stop on driveway in the dark with lights on and foot on the brake, I can notice how head lights are dimming at those short moments. Would that happen with some transmission problem?
Scan the car for codes first and don’t assume the plugs are good because of the mileage and their appearance.
I just ironed out a badly skipping car due to ignition misses and the problem was the Platinum plugs. They looked perfectly fine, were burning normal, etc. and only had about 10k miles on them yet they were still the cause of the problem.
This is especially true with plugs that use wide gaps. The wider the gap the more prone they are to problems like this.
Personally I would have already changed the plugs. They are cheap and in most cars easy to change.