Engine loses power during warmup. Hyundai Excel 1996

what year car< 1994 to 2022+ sorry can't guess 28 years of car evolution.
in what country.
what engine is there losing said power , non told.
so lacking all useful facts lets go generic, (gasoline only engines)
USA only 1996 and up.
1: transmission fails to up shift, scan it.
2: engine spark is bad, misfires power loss, time for a spark and all filters turn up? 100k mile tune up scheduled skipped?
3: fuel bad, fuel pressure too log or too high , lost power.
4: sensors on engine failing , SCAN IT. MAP or MAF.

one of them.

similar problem was caused by a blocked fuel filter. after running for a while, the dirt in the filter is drawn against the filter paper blocking the flow

Sounds internail, Hopefully it's just maybe a rod bolt come lose. But could also be the camshaft, Check and see if your still getting fire to plugs, If camshaft is busted it will not fire. Befor you have any work done be sure to get a sec, opinion and price quote. Best of luck to you.

check ignition coil current supply and or distributor contact point or condenser/ ur cars current supply fluctuate when engine is hot....

sometimes loss of power can be caused by a clogged catalytic converter which can make the car at times hesitate, and about the trasmission maybe the governor on it might be faulty what that does it changes the gears when it reaches certain speed

sounds like you are low on gear oil

hi-- i believe they are referring to the ignition timing not the valve timing!-----however,excels are bad for doing timing belts and if you have done the kilometers --then by all means have the belt replaced--------it can cause very expensive damage if it goes on the road---cheers

crank angle sensor for sure

sounds like a transmission slip sepecially if its an automatic go to a shop and get it checked its not an easy fix