Need Help With My 89 Jetta

I have an 89 jetta like the title says, but a couple of weeks ago i went to see my friend, car started alright went there and when i was leaving, and i tried to start the car, i started it in gear, so like usual it jerked a little bit. but after that the car wouldnt start anymore and i had to get a push to start it. i dont think its the starter, because its a fairly new one. someone suggested i knock with something, because it might just be locked or something, but i didnt think it was the most brillian idea. then just today i was talking to one of my friends and he told me could be an electrical problem. so then i remembered that there was one dude in this place that was saying something about some sort of diagnostic thing which can pretty much detect mulfunctioning parts of your car and those that are bound to mulfunction soon. so if anyone is in here that can give me any advice as to what to do next, it would be really appreciated, i love this car and i'm willing to do pretty much whatever it takes to bring get it working, ie not having to push it to start. And one more thing, the rpm sometimes stays above the required level when the engine is hot i think, the only way i try to get it back it when i rev the engine then it comes back to normal. And also for some reason it has a slower pick up rate, compared to my 89 accord, which pretty much has the same horse power. but thats besides the point, the main problem is with the starter, so please and help would be really appreciated.