Well, I'm at a loss; I fear this will take someone with more knowledge than I to cure. All my testing has produced inconsistent or inconclusive results.
To recap:
The GP37 cars, and the TR3 mod, have worked fine at other tracks I've tried; at Clermont65 the TR3s do not move at all, the GP37 cars may get on the track, they may move very slowly to the pit lane and eventually move normally, they may constantly move back and forth a foot or so, or they may back up in a stop and start manner til they are half buried in the garage, where they will sit for a minute before disappearing, and after a couple of minutes reappear in their normal pit position ...to repeat the scene. I can give these cars a little push and they may move out slowly before speeding up normally, or I may push them nearly onto the track before they come to life.
Tried some default cars. The Ferrari 360 Modenas had no problems, they all zipped out of the pits and onto the track. The Vertigos got on the track, though a couple were rather slow, and one (in the space closest to me) just sat there, didn't rev the engine, didn't turn the wheels, might as well have been scenery. I maneuvered behind it and pushed (wasn't easy, as if it weighed two tons and had the parking brake on), it did nothing til both front wheels were actually on the track, then it took off normally.
I will point out that the cars that do get on the track, whether on their own or with a push, often often zip across the pit lane and partially onto the track, then pull back into the pit lane and slow to normal speed.
I have tweaked the idle settings and played around with torque figures in that range (keeping in mind another thread here about the lowest 3-4 lines needing negative values for torque or neither the player nor AI can make pit stops, since the engine will not shut off). Also tweaked the garagedepth setting, as noted in previous post, but results are inconsistent. I checked the values of this in numerous other tracks and decided the setting itself seems irrelevant; most default tracks have this between 2.000 and 3.000, of the add-on tracks I checked some were in the 2.000-3.000 range, the rest had no garagedepth line at all. Solitude is one of the latter, yet all cars I've run there, default or mod, work fine.
Here is a shot of the pit area at Clermont65; taken at the end of my third lap, so probably 11-12 minutes into the session. Cars line up on the right facing the track at an angle; they should pull into pit lane (the light area just to right of the cones) and proceed to the far end where they enter the track in turn 1. The Auto Union has merely moved back and forth til it is sitting at a much greater angle than normal, while the Alfa backed, in fits and starts, into the garage (it vanished a few seconds later, and reappeared in its pit space about two minutes later). The car in the space this side of the Alfa is on the track ...after i pushed it. The car this side of the Auto Union got on track, after meandering back and forth very slowly for a couple of minutes; the car by the light pole has just vanished, after backing halfway into the garage.