Iggy,
Like Raywood said it's probably the battery. When the battery gets low it will do as you described. Charge up your with a "battery tender" and check the battery voltage with the engine not running, should have 12.8 volts or 13. something. Then start the engine and check the voltage with the engine running. You should get about 1/2 to 1 1/2 volts more on your meter which will indicate the charging system in doing what it should. You can also increase the rpm and see if the voltage increases also. Let us know how it comes out.