engine sputters after warmed up

My 95 teg just developed such a problem after I came back from a 3-week trip and started driving it. I didn't modify any part of this car. When it's cold, everything is fine, but after I drove it for about 6 - 7 miles and the engine got warmed up, it started to sputter at normal speed (below 50 miles/hour), when it's above 50, then it's ok. The idle also got bad, it would drop and then came back to the normal rpm of 750. I felt it was about to stop when the idle speed dropped and the car would shake at the same time.

What could be causing such a problem? Where do I start looking? Thanks in advance.