Cooling Question

I recently purchased an 84 280GE LWB. The vehicle has an aftermarket A/C unit. I've noticed that during some of these hot summer days, the vehicle runs hot when the A/C is on- by hot I mean the temp gauge needle comes very close the red zone. I should note that most of my trips on these days have not been long enough to give the gauge a chance to go into the red. This is, however, a bit disconcerting. I am aware of what this can signal with other types of vehicles. Is this common for G's of this vintage, or do I need to be concerned.

Apperciate any comments or suggestions.

Otherwise, I love my vehicle.