Onix A-120MKII Gets hot fast.

I'm not familiar with your speakers but it sounds like your amp is going into protect mode. Reading a review here
http://www.positive-feedback.com/Issue20/onyxrs550.htm

they had some problems driving Theil 2.2's. It may just be reaching it's power limit? A higher current amp may be necessary? Someone else may be more familiar with your speakers and know for sure? Remember the watt rating of your amp is not really important. It's the current it can provide to the speakers that's important. "amps power supply"

Current capability isn't something you can check per-say. A good indication of an amp's current capability is how much power it has at 8 ohm's say 200watt and close to 400 watts into 4ohms. This is not an exact way of checking but it is a good indication of the amps power supply. The closer it comes to doubling it's power the better.

If you have another pair of speakers that are smaller and less demanding give them a try. If the only time the amp over-heats and goes into protect mode is when your pushing it. I would guess it's just running out of gas.