Transient rpm drop and high oil temps on takeoffs

Well, you have reoccurring debris in the fuel.

The RPM loss and Oil temp increase could be a byproduct of marginal fuel starvation. Lean burn.

You have new fuel lines. NEW is not the same thing as clean.

I would open the fuel lines at the carbs and flush a liter of fuel from each hose into a clear container.  Debris?

Then remove the fine fuel filter and do the same thing.

Install a new Fine filter and reassemble all hoses.

If the problem reoccurs and you find more debris in the cab bowls;  it is originating After the Fine filter.

Dear Adam

What material is your fuel tank made of?

I will forego the detailed description of troubleshooting.  The short version is that after Bill's suggestion I checked the fuel pump strainer (clean) and then removed and backflushed  the inline fuel filters (each of the four tanks outputs via a 70 micron steel mesh filter) and replaced the Andair gasocolator screen after seeing how much sediment it had accumulated.  Note that I flushed the lines on renewal, but since then have run something like 150 gallons of mogas and avgas through them.

An astonishing amount of sediment was present.  In the photos attached (really!) you can see the mass of what appear to be rubber and other fragments were were flushed from the four inline filters (white coffee filter) as well as what washed out of the gascolator (blue shop towel with a 13.3 Oetiker clamp in the background.

I believe that the marginal fuel starvation hypothesis is correct.

Today I completed five departures with normal conditions prevailing.  These were up to pattern altitude; when the weather is better I'll go higher.