Lower power being a relative thing, it seems like my engine has more power
when cold than when it warms up (or begins to get too warm).  Why is that?

When it warms up I also get a rougher idle, that can often be solved by
restarting.

I have a 85 auto Westfalia with a tired 1.9 (275k km on original motor but
new heads twice I think).  Just biding my time until it's New Motor Time and
thinking about my options.

Thanks
Jeff

85 Westfalia GL
69 Type III notchback