420 SEL idle surge

[Carlo(s)]

After new timing chain + rails + new oil + filter & valve seals car was idling smooth and had good power for about 20 miles; filled tank with regular gas and a slight rough idle appeared that is getting rougher and engine is down in power; car surges 300 RPM up and down at idle; no starting problems hot or cold; all plug wires cap & rotor new in the last 10,000 miles; checked all Bosch Wd9 Super R plugs that have a about 20,000 miles and all have strong spark, cleaned them out and re-gapped at .42 to no avail. Fuel filter is about a year old; new bosch fuel pumps within last year new injectors and injector seals + air distribution tube. Any suggestions would be appreciated.