1969 volvo amazon B20A engine issue

[Jens Vermeulen]

Hi

About 1,5 years ago i bought a well restored volvo amazon.
It has recently developed a engine issue.
I can best describe the issue as the engine holding or pushing back when accelerating.
To me it seems that the fuel mixture is igniting before the cylinder is at top dead center.
The engine can be completely fine one day and the next day this issue can occure.
I often drive my car (sits for max 2 weeks)
Things i have already done:
Replaced oil
replaced condensator and spark distributor (low spark issue)
this last item was done by a garage, they also tuned the pre ignition
after that it was fine again but reoccured after a week or two.

Any suggestions?

Thanks in advance