UPS compatibility with Studio XPS435T

When the battery on my older APC battery backup unit (a BK650MC) died last month, I bought a newer model of BBU, an APC BE750G for my 34-month old desktop system, a Dell Studio XPS 435T. The new BBU has introduced major problems and I'm trying to determine why. It self-tests periodically on its own and I can cause a self-test manually; at the point where the BBU cuts over to the battery, my PC always loses power for a second or so and reboots. A couple of possible causes are that (1) the BE750G lags in switching to battery; or (2) the Dell requires a near-perfect sinewave power form and the BBS produces only a modified square wave; or (3) the Dell power supply has lost its capacitance ability to bridge a ~5 millisecond blackout.

I'm also discussing this with APC, but I'd like to know what the specs are on power-quality requirements for the XPS435T; how long should the power-supply capacitor be able to take a brief outage, and how finicky is it about the approximation to a sine wave?  And is there a way to test the power-supply capacitor via diagnostics?