'01 Tiburon Tranny Problem!

[scarlet21]

'01 Tiburon 112,000 miles.
OEM Codes: P0734 & P1624

In May while merging onto the interstate I could feel that the car did not shift into a higher gear. Sure enough, when i got up to 45 mph the car was reading 3-4000 rpms and making a rather low sound and felt like it physically wouldn't go over 45mph. I pulled over and shut off the car. When i turned it back on and merged back into traffic the problem was fixed. Fast forward to the 4th of July, again while merging onto the interstate the car didn't shift into a higher gear and this time the check engine light came on. I pulled over as soon as i could and once again it fixed itself, however the light remained on. Since all the car places where closed I waited until the next morning to take it in. Of course on my way there the engine light turns off. They drove around the car and basically told me that they don't know what is wrong with it because it was driving fine. They told me they could throw some parts at it but until whatever it is actually goes out they aren't sure. They said it "might" be electrical and to take it in while it's messing up. Well ok, that was last week and tonite while driving it, and merging on the interstate, the check engine light came on FIRST and then it messed up. On my way home the car drove fine and the light turned off. What is wrong with my car!? How much will it cost to fix? Is the dealership the only one that can fix it? Also, should i take it into the shop tomorrow anyways even though the light turned off and the car seems to be fine again. Thanks!

'01 Tiburon 112,000 miles.
OEM Codes: P0734 & P1624