If anyone still intersested in running Apache vs Havoc in it's vanilla form, I've found a way to do it without that annoying speed fluctuations.
I've posted the solution on GOG forum
https://www.gog.com/forum/enemy_engaged_series/inconsistent_display_speed/post12I was having this issue and tried to resolve it few years back but nothing helped.
I've tried running the game on one CPU (with imagecfg.exe) along with CPUgrabber and CPU killer (running on the same core as the game). It helped in some ways but there was still occurences when the game became choppy with inconsistent time speed in game. BTW I always try to resolve any issues without having to use any CPU bogging software as it stresses the CPU which isn't an elegant solution.
I've tried enabling the vertical sync to no avail.
I've also tried decreasing the multiplier on my motherboard for the CPU from stock 15x to 5x making my CPU basically a 1GHz running chip (It's running on 200MHz FSB -> 5x200MHz=1000MHz), but that also didn't helped. So I finally gave up.
I recently got interest to play the vanilla game again and tried simple solution, I don't know why I didn't tried that earlier. Basically what I have done was to limit the FPS for the game. That made the game run smoothly, without any inconsistance in speed! Just like the old times. Unfortunately, the nVidia software doesn't provide any means to do that so I used nVidia inspector to limit the FPS to 30 and forced the vertical sync (see attached file).
I've tried that on Win XP SP3 with GTS450, but will have to try that on my other, less ancient PC with Radeon HD 6850 and Win7.
I hope this helps anyone who wants to try the game in it's vanilla form.