HT-Dual muffled tone

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Hello,
I have a Ht-Dual since a few years.
When I use the pedal, sometimes the tone is going muffled.
To fix this problem I kick the pedal with my foot and the tone goes to normal (more brighter). And after a while, the tone is again muffled...
Do you know how can I solve this problem ?

Thanks, regards
Stefan