engine hasitation

hello guys:
I have some problem with my little Renault clio 1.4 RN（ｆｒｏｍ１９９３）.
I　ｊｕｓｔ　ｂｏｕｇｈｔ　ｉｔ　２　ｗｅｅｋｓ　ａｇｏ，ａｎｄ　ｆｉｒｓｔ　ｔｈｉｎｇ　ｉ　ｄｉｄ　ｉｓ　ｃｈａｎｇｉｎｇ　ｅｎｇｉｎｅ　ｏｉｌ，ａｉｒ　ｆｉｌｔｅｒ，ｓｐａｒｋ　ｐｌｕｇ，ｆｌｕｅ　ｆｉｌｔｅｒ．　ｍｙ　ｃａｒ　ｗａｓ　ｇｏｏｄ　ｂｅｆｏｒｅ　ｉ　ｃｈａｎｇｅ　ｔｈｏｓｅ　ｓｔｕｆｆ．　ｂｕｔ　ａｆｔｅｒ　ｃｈａｎｇｉｎｇ　ｔｈｏｓｅ　ｔｈｉｎｇｓ，　ｗｈｅｎ　ｉ　ｓｔａｒｔ　ｍｙ　ｃａｒ　ｅｖｅｒｙ　ｔｈｉｎｇ　ｓｅｅｍｓ　ｔｏ　ｏｋ，　ｂｕｔ　ａｆｔｅｒ　ａ　ｗｈｉｌｅ　ｉ　ｆｏｕｎｄ　ｔｈａｔ　ｉｎ　ｉｄｅａｌ　，　ｔｈｅ　ｅｎｇｉｎｅ　ｓｐｅｅｄ　ｗｉｌｌ　ｓｕｄｅｎｌｙ　ｇｏ　ｄｏｗｎ　ａｎｄ　ｂｅ　ｃｏｍｅ　ｎｏｒｍａｌ　ａｆｔｅｒ　ｆｅｗ　ｓｅｃｏｎｄｓ，　ｂｕｔ　ｎｅｖｅｒ　ｓｔｏｐ．　ｗｈｅｎ　ｉ　ｄｒｉｖｅ　ｉｔ　ｓｐｅｃｉｓｌｌｙ　ｉｎ　ｆｉｒｓｔ，　ｓｅｃｏｎｄ　ａｎｄ　ｔｈｉｒｄ　ｇｅａｒ　ｙｏｕ　ｃａｎ　ｆｅｅｌ　ｌａｋｅ　ｏｆ　ｆｌｕｅ　ｔｈａｔ　ｍａｋｅｓ　ｃａｒ　ｓｈａｋｅ　ｂｕｔ　ｉｎ　ｆｏｕｒｔｈ　ａｎｄ　ｆｉｆｔｈ　ｇｅａｒ　ｉｔ　ｄｏｅｓｎ'ｔ happen.it only happen in 1,2,3 gear. that makes me little but worried about my car . so can please give me some advise . some body told me it might be a problem of the sensor in the car , in that case i have to go to check it in computer . but i have no idea what to do .
I am waiting for your reply . more information up on request.

thanks very much !
best regards
sardar