2000 Hyundai Sonata

I have a 2000 Sonata with around 150,000 miles - I had the radiator replaced and about two weeks after, the car started making a mental on mental noise after I have been driving for about 5 minutes and after in goes into second gear only. I took it to a repair shop where I had the front brakes done, the rear brakes done, a tune-up, an oil change, new fluid filter, new air filter, main motor mount replaced and everything tighten, but still it is making the same noise and still hesitates when I driving every now and then.. No one can seem to help me and it has already cost me around $1800 to do the things I have done already in the past 1 1/2 months. Can anyone help me...