(once DirectX is upgraded, you can't just downgrade).
DirectX Eradicator to help you.
I suspect the game demands a lot of CPU power,
No, the performance hasn't really changed much, either from going from the P4 to the x6800 or from the x6800 to the i7 4790k. Mostly depends on the video card and driver.
is there a fps counter for this game?
Fraps will do.
Tried to revert to dx7.0a, old Nvidia drivers (4.14.10.8185) but nothing seems to work.
Any suggestions?
The ForceWare 77.72 for Win9x driver is the latest relatively hassle-free for this game, using an alternative DX7 handler (dx7z.dll), of course, since the original "dx7z.dll" requires ForceWare 56.64 (for Win9x) or lower at most. Obviously "old Nvidia drivers (4.14.10.8185)" is the problem in your particular case. Also, as an option, you can try playing with Glide or OpenGL emulators.
Porsche unleashed. It will start without any issues, then at the menus it will freeze. The music keeps playing, but the mouse pointer will not move and no keyboard combination (Ctrl-Alt-Del, Alt+F4, etc.
This is a well known issue and only affects Windows 9x\Me. To solve it, immediately after launch, minimize the game and lower the priority using any third-party task manager, such as Process Explorer for example or similar, or immediately start the game using a batch file (BAT), which will specify a program that automatically lowers the priority for process "PORSCHe.ICD". For example, I am using the product "PV.EXE" for the specified purpose. This is how it looks like in the batch file:
start /MIN "E:\GAMES\NFS5\Porsche.exe" /MIN
timeout /t 5 || ping -n 6 2130706433
E:\GAMES\NFS5\pv.exe -pi Porsche.exe
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
E:\GAMES\NFS5\pv.exe -pi PORSCHe.ICD
It is written several times to ensure that the priority is guaranteed to change (Quite often PORSCHe.ICD does not start immediately, but the program should have time to work out correctly). Or, as already mentioned above, the profile for this game in RivaTuner makes it look like the same thing in principle (Did not check).
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As for the issue of reduced graphics quality on >2000Mhz processors, I'm not completely sure as I'm playing the original release version 3.0 installed from CD without any patches, but the first time after launch I had to go into the graphics settings menu and enable everything to the maximum. This needed to be done once.
Further, as far as I remember, I played with various third-party patches designed to improve graphics, but they either did not give a significant effect or lowered the FPS too much, so in the end the completely original version was returned and successfully completed without any significant problems or crashes .