There's three basic keys to a successfully starting engine, check the following before you take it to a mechanic:

- Adequate battery power (you've ruled that out noting that it cranks)
- Ignition/spark (pull a plug wire off and see if you have any spark - be careful)
- Fuel flow (check the fuel filter, sounds like inconsistent fuel flow)

Aside from that, I also ran into a similar issue where I had assumed that the car had a broken temp guage - come to find out, that wasn't the issue, it was just running cold (that's right, cold!). A quick thermostat replacement and the car ran with no problems.

Also, don't underestimate the impact that dirty battery terminals can have. Clean them if they look like crap. Check the distributor cap and rotor (if they haven't been repaced lately). Fuel pump could be bad, but usually they either work or don't, they're usually not inconsistent.

Rule out the basics first.