Preamplifier produces glitches - Help needed

[Guest]

I have Realistic STA-2000. The amplifier section forks fine. The TONE AMP however produces random (very frequent) glitches with maximum power output. It sounds like cracking or like when you have short circuit. The glitches are visible on the output power meter. Visually there is nothing that suggest which component might be wrong.
Any help is more than welcome