The lasing of a Van de Graaff electrostatic accelerator - free-electron laser (EA-FEL) with an internal cavity is reported. An EA-FEL employing an internal cavity is a FEL configuration that has a potential to operate at high average power, high frequency, and possibly in a continuous wave (cw) mode. The initial lasing provided a pulsed radiation power of 1 kW at 100.5 GHz frequency. The FEL operated with a 1.4 A, 1.4 MeV electron beam in a recirculation (depressed collector) configuration. It utilizes a high quality (Q≈30 000) Talbot effect resonator and a Halbach-type wiggler placed internally in the center of the accelerator tank. Nonlinear features of the oscillation power buildup and decay near saturation and mode hopping were observed and are interpreted.