rpms sticking and jumping

In an reply to the below question i had asked, after it died and i added oil i took bit to a mechanic who did test and said i was really lucky and had no leaks at all, he even kept it a second day to test it and there was still no leaks, it worked better than before i bought it for the 2 days until today. Also, the coolant leak is apparently in the resevoir from high pressure from the overheating.



For the last 2 weeks, my rpms were sticking and jumping, my car was over heating and i had a coolant leak.this weekend, i ran my engine completly dry of oil, it died, and i replenished the oil and it died again. i then changed the oil filter and my car ran perfect, better than before.it ran perfectly for monday and tuesday.today, my rpms have started to "stick" again, meaning, i stop accelerating, or the car has gotten up to speed, the rpms do not lower. as well, they are riding very high,,,,ex normal doing 40km would be 1.5-2, now is running 2.5-3.as well, when i am slowing down and go below 40, or start to come to a stop, the engine sort of "jumps" and the rpms sort of jump and lower.im terrified it will break down again.before every drive i have checked the oil and coolant and both are full.any suggestions would be so greatly appreciate