bmw e36 model not running properly, pls help! :(?

My e36 323i bmw is very sluggish and lacks power whilst driving, also the car starts chugging, this seems to worsen as the car heats up. I have tried renewing the plugs, removing the old air filter and changing the coil packs but nothing so far has made a difference. I like the car but i am prepared to sell it if i cannot find the fault. so if you know the cause or have any suggestions on other things to try pls help!! :(