1997 Toyota Camry RPM meter Jumps

I don't know very much about cars so please explain as much as possible. The problem varies. Sometimes when I start my car the rpm meter will start at the normal idle, then violently drop and when you sit in the drivers seat you can hear the engine dying. After the rpm meter drops down it returns to normal rpm and it repeats this process until the accelerator is pressed and I go. Several times at lights it will do this same thing, it has even died at stop lights. Whien it dies the breaks are very stiff or hard. I have to press with all my might to get the car to stop. Somehow, gunning the engine up to about 6,000 rpms or so seems to clear the problem if the car is still running. The first time that it did this was after a long trip on the highway. And then it proceeded to do this in city driving so I went to a mechanic they scanned the car with their computer and no codes came up. They proceeded to do a tune up on my engine, which didn't work. I then went to another mechanic in town, because the first proved themselves to be less than competent. They scanned the car again and said that there was a "slight" code on the EGR valve. They then proceeded to tell me that it wasn't severe enough to warrant a replacement of such a part because the code wasn't sever. How do you have a sever or slight computer code? Anyway, the car was bought used and they sold it just before the 60k service. Naturally I got the service done and then the problem stopped for a while until one day I went on a long trip on the highway. I stopped at a rest stop to get a break and whatnot but the car started to do the rpm jump again. I gunned the engine once again and it cleared the problem. It hasn't done that again that dramatically but the RPM meter at idle is never solidly on 1 tick mark or the other like other cars that I've noticed. I hope you all can help me with this problem. I don't want it to happen again and not be able to start my car in the middle of nowhere. I also just recently had an oil leak fixed. I don't know where it was leaking from. Could this be related? Additionally I've also just had my 75,000 service. I dont know what they did but it hasn't changed the problem. Thanks for all of your help.