I have examined your circuit and have some questions and suggestions. But first, some house keeping.
You should have labelled the components in order to make it easier to discuss by post. So I have labelled the transistors Q1 - Q6 where Q1 - Q4 correspond to G1 - G4 and Q5 & 6 are the upper and lower transistors of the "half bridge DC- DC converter". I also labelled the rectifier bridge RB1 and the 220 uF capacitor C1.
Questions
1. I assume the transistors that are over heating are the Q1 - Q4, not Q5 & 6. True?
2. I assume the PIC generates PWM signals that, in conjunction with the MOSFET driver circuitand the 0.01 uF capacitors across the Vb & Vs pins of the MOSFET drivers somehow create a sinusoidal drive signal for Q1 - Q4. True?
3. You said that when the voltage reaches 100 Volt, the "current spikes" and the transistors overheat. I assume this voltage is measured across C1. True?
4. If 3 is true, then I assume that you disconnected RB1 and applied a variable voltage source to the drains of Q1 & Q3. True?
Conclusion
If 1 is true and my other assumptions are true, then it would appear that transistors Q1 & Q2 and Q3 & Q4 are all turning on when the voltage reaches 100 Volt thus causing an excessive current through them and causing the overheating and "current spikes".
Have you looked at the gate waveforms with an oscilloscope? This would show whether Q2 & Q3 are off when Q1 & Q4 are on and vice versa.
Len