need help! car is actin up!

i need help on diagnosing my 91 LS SPECIAL 5 SPEED.
for some reason…car is stalls. when you start the car its like its going to die out but when i step on the gas for a lil bit…its going good and idling perfect…but when you start driving it and shifting…the gas pedal or when you step on it …its like its hard and you really hard. and when you are on 4th or fifth gear.its having a hard time to pull…what could be the problem…i did put a new spark plugs. new battery…any help would be much appreaciated.