Not sure which part of the throttle mechanism would cause the thermal expansion:
Ferritic steel ( the material which the linkages are made from in the original design falls in this catergory) expands with 1.2 mm/m/100 degr.C.
So a linkage of say 100 mm and working at 70 degr.C will expand only 0,06 mm, assuming it was adjusted at ambient temperature, 20 degr.C.
The aluminium components however expand at a much higher rate, almost double of ferritic steels (around 2,2 mm/m/100degr.C.
You can check your design by estimating how the respective materials expand.
Have you set the float (clearance in closed position) correct on the linkages?
When I collected my reconditioned PI equipment the reconditioner supplied a different linkage kit to the original, this meant that following the manual wasn't any use. There really isn't any adjustment possible other than the throttle rod length, I have been setting these such that they fit without opening the butterflies with all the parts warm, although not at full temp.
I started the car earlier, idle at 1100 RPM, within a few mins the exhaust manifold was up to 236C on my infrared thermometer and the idle was up to 1500 & rising.