Volume breakdown and surface flashover in quasi homogeneous applied fields in 10-5 to 600 torr argon are investigated, using voltage pulses with 150 ps risetime, < 1 ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of one to several mm. The arrangement on the other side of the gap is symmetrical. An improved system, with oil-filled transmission lines and lens between coax and biconical section to minimize pulse distortion, is being constructed. Diagnostics include fast capacitive voltage dividers, which allow to determine voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics uses a scintillatorphotomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics to obtain information about the discharge channel dynamics is in preparation. Breakdown delay times, and e-folding time constants for the conduction current during the initial breakdown phase, are on the order of 100-400 ps, with minima in the range of several 10 torr. X-ray emission extends to pressures > 100 torr, indicating the role of runaway electrons during breakdown. Maximum x-ray emission coincides with fastest current risetimes at several 10 torr, which is probably related to an efficient feedback mechanism from gaseous amplification to field enhanced electron emission from the cathode.