I am the new owner of a 1972 914 with a 1.8L engine and L-Jetronic fuel injection. The car runs but begins to stumble when I reach 3,000 RPMs. Where should I begin looking for the cause? Thanks in advance for any assistance.

Start with the basics-
Spark plug gap
Timing
Fuel Filter
Condition of the plug wires
Clean your grounds

Fuel and timing is where I would start.

Can you duplicate the stumbling while revving the engine when the car is in neutral, or is this only happen when driving?

we all talk about the basics, but the trigger points need to be checked as well