Is It My Clutch

[rush]

Hi

can someone help me im new here and this my first ever toyota i bought. i recently bought a celica t- sport 190 bhp vvti. i always liked the shape of these and decided to part my hard earned cash . recently when driving it seemed like the engine is struggling to accelarate . im doing 4th gear with a rev of about 4 to 5 and i can sense the engine is struggling to pull. a garage i spoke to said that it might be the clutch slipping . the clutch is very stiff and there is somtimes a small knocking sound whn driving. also when i try to pull off first gear i stall. sometimes its ok. can clutch slipping be the cause of this high rev in low gears and engine lagging. if i change the full clutch kit would this work. is there a test i can do to find out if its the clutch

thanks