Peugeot 405 1.9 td 1995 Oil in water.How can I fix this mystery?

A couple of weeks ago I noticed some oil in top of rad. This was clean oil (no emulsion there or inside oil filler cap).There was no over pressurising and heater worked so I thought it may not be head gasket.This also coincided with a couple of gauges ceasing to work.
I thought it worth trying another oil cooler so I've just fitted a replacement (used item). I've flushed out rad. and I'm going to run it around locally with a couple of 'radflushes' in there and intend to repeat that a couple of times to try to clean the system.Am I wasting my time? Any suggestions? Thanks.