1999 Deville engine misfire no codes

I have had this problem from last year but the car has been off the road since November here in UK. At start up the engine runs fine for a short while, then begins to misfire, driving is ok, the car recovers but with lack of power for while, then runs fine. Idles uneven miss? Parking vehicle engine runs uneven, lack of power, and when restarted later runs uneven whilst manoeuvring, then lacks power for a while. The engine then settles down full power. I have to operate accelerator gently otherwise engine stumbles with lack of power. At idle engine is uneven, but rev engine and it has good power after a couple of splutters. I changed plugs last year to Bosch, these are fitted now. These are correct, checked with Rock Auto. I did try AC Delco plugs from Rock Auto, but these made the situation much worse with misfires and backfires on the road. I have no engine service lights and no current codes. Many thanks