1986 Mercury Marquis runs rough especially during accelerat

This symptom starting occuring suddenly. The engines seems to run a little rough but is most noticeable when coasting at 20mph then pushing on the gas pedal a little, at that point the car seems to shake a little and it sounds like the engine is running rough, and maybe a slight power loss is felt. If you continue to accelerate it seems to smooth out. It will do the same thing when coasting at other mph's then accelerating but it's not near as pronounced as at 20mph. I think I can tell the engine is not running as smooth as it should when just sitting in drive and stopped, but it's hard to notice. It seems most noticeable at any time when first pushing down on the gas pedal. Any help appreciated.