Well, your right, it could be quite a number of things.
Let's start with the basics....
Was/is the check engine light on at all?
When was the last time that the plugs/wires/filters were changed?
When was the timing belt changed last?
Any guesstimate on how many miles on the engine?
Any odd noises while it's running?
Generally, what you describing could be a lack of fuel or ignition related problem.
When you open the throttle does the engine struggle or sort of gasp and try to die?