Should I remind someone of a previous invitation to give a talk?

It's certainly reasonable to send another email just saying that you would really like to visit, and that you would like to take up the old invitation. By far the most likely thing is that the professor in question just forgot, and will appreciate being reminded.

Since several months have passed, it is more than reasonable for you to email the professor once again. I have been in a very similar situation, and the professor was very cordial in his response and apologized for his lapse in memory. Professors are usually quite busy so they will not be offended if you restart the email exchange. Don't worry one bit, you'll be just fine!