Hi Paul ,
Id have a look at the HP400 manual ,
theres lot of good fault finding tips and spot checks that can help determine the cause of problems .
My HP4378 has settled down very nicely over the last few days being powered on ,
it reads very slightly high on ohms range compared to the fluke 187 ,
51.211 ohms vs 51.15
I read in the manual that HP cal is performed at 23 degrees C and ambient indoor temperature here now is only around 15 C , they speicfy a range of +/- 5 degrees C ,which Im obviously outside.
so anyway I'll need to wait until the weather warms up a bit before I taking a trustworthy reading .
One word of warning about precission test gear that uses dual or triple section caps in the PSU , they may be relying on the precise physical properties of that cap to meet the noise performance or other spec , the metal can and concentric wound elements can provide sheilding and screening that discrete modern components cant ,
The Mallory brand metal can multicaps seem like the modern equivalent to whats used in both the Tek and HP units I have , unfortunately most now come with fast-on terminals , not PCB mounting .