Difficulties in biasing MOSFET push-pull amplifier
The problem is that your creating what amounts to an amplifier output stage, but with no feed back or voltage reference. Please look at the designs of audio amplifiers or op-amps and you will see a differential input stage.

The inverting input sets the gain of the amplifier and the (+) or non-inverting input sets the reference voltage, or at least that's where it is applied. In this case the ref voltage would be 1/2 your supply voltage.

Typically the audio signal is fed into the (+) input and amplified by a small amount at the differential input stage (2 NPN transistors). This drives a level shifting stage (your BD140) which has high gain and this drives the final output stage you have shown.

I am showing you (sorry for the poor quality. Its worse if I enlarge it) a MOSFET output 500 watt amplifier. Forget about the 500 watt rating an focus on the design. The input is a differential amplifier made with 2 PNP transistors. The MOSFETS have gate resistors of 20 to 33 ohms or so to prevent ringing You see the output has a feed back resistor going back to the differential input. The ratio between it and the resistor from base to ground controls the gain. Because this has a bipolar supply, the (+) input is biased to ground with a 100 K resistor. The feedback resistor can be 22 K, and the resistor from base to ground can be 2.2 K through a 47 uF cap, to block the DC voltage from being grounded, giving you a gain of ten.

In your design with a single ended supply you would bias the (+) input with a 100 K resistor to the + supply rail and a 100K resistor to ground. This sets the bias voltage at 1/2 Vcc, so your output will try to stay at that point. This has nothing to do with the MOSFET gate bias. Normally your 330 ohm resistors would be .33 ohm 5 watt ceramic resistors. You measure the voltage across them until the idle current is 50 mA. You MUST use a heat sink if you plan on using .33 ohm resistors.

Because you have a single ended supply your output will be 1/2 of your supply, so to keep DC from going to the speaker you would use a 470 uF 50VDC capacitor at the out put, with the caps (+) lead going to the MOSFET center point between the two resistors.

Even if you don't build it into an amplifier, the addition of a differential input will keep the output voltage very close to 1/2 your supply voltage.

Your design is unpredictable because it drifts by temperature and the gate voltage. Without feedback from an input stage the output has no sense of where it should be (1/2 the power supply voltage), even with your gate bias resistor set properly.