If these are built-in then perhaps this isn't the problem... Now that you mention the little detail here, the fact that you have, gulp!!!, 75-80 drivers, this might be the problem. Your computer/game is running out of resources when it attempts to track these drivers. Keep in mind too that the program is also tracking all the switches (junctions) signals, interactive industries, and all kinds of other messages the assets are producing.
It could be that these last few drivers are at the tipping point of being just too many and that's when things are getting a bit wobbly. If I were you, I'd clone the session and try eliminating a lot of the drivers and see if that solves your problems. Remember no matter how much memory your computer has, and how fast your CPU is, there are still hard limits somewhere that will eventually be reached.
If you want, enable logging. In TS12 you need to look for the jetlog.txt file. This will display any errors, and other messages, with what's going on. For T:ANE you need to click on Developer, on the Launcher, then pick Show log. This doesn't save the text like TS12, but instead displays everything in real time. You'll ne amazed with how much information gets passed around between the various components in Trainz, any version, for that matter.
By far your 75-80 drivers is about the largest number of drivers I have ever seen.
John