Without knowing more details then what you have stated here is what i would look at for starters to pin down the problem
When was the last time the engine had a good tune up
What does the roter and distributor look like
Have the exhaust sysem checked out to make sure it's not plugged
Check the fuel filter to make sure it's not plugged Check air presure in the tires
Is the air filter clean or do you have a spare tire in it's place ...LOL
When was the last time you replaced the coil