After replacement of the ignition switch unit and tumbler on 126 420 SEL the tach needle flickers between 1-2000 RPM even under acceleration; there is no power loss or misses so engine is running fine but tach needle flickering seems to be getting worse; what causes it? where is the tach sending unit? also it appears that engine oil pressure needle quit functioning; it pegs to the top immediately upon turning on the ignition switch. I remember glancing over a few threads on this matter in the past but can't locate them. Instrument cluster been out a couple of times and tach wiring harness seems OK; no loose wires anywhere; no misplaced connections anywhere. Everything else in the instrument cluster including lights, etc. functioning properly; just tach needle flickering and oil pressure pegged at 3.