Yesterday while going back to home, my Civic 88 (D-12 Carby) started Ghrrrrrrrrrrrrghrrrrrrrrrrr while driving for about 5 to 10 minutes, then became normal by itself. It felt like loosing power and gaining back again. My car is on petrol since last two years, cng cylinder is removed, so no question about it. Kindly tell me the reason and its remedy please. I must add that its fuel system is original/mechanical pump.