is it only happening with the R4 cartridge or does it do it with other game cartridges as well - sometimes the R4 needs padding (paper or thin cardboard)behind the PCB to slightly raise it up so it makes reliable contact with the pins
otherwise check for bent or misaligned pins inside the connector
if ok - try cleaning the copper contacts of the slot 1 connector
what version R4 - if its the older ones that came with both white and brown coloured cases you can use the spare case, open it up and put a rectangular piece of super fine grade wet and dry sandpaper with some thin cardboard behind it to space it out
insert and remove several times to polish the connectors
failing that i can replace the slot 1 with a new one, or fit a replacement board
regards...
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