IBM Model M - Inconsistent Lshift+Y and Rshift+M

[HenkHeukers]

Very weird problem with this model M I just got.
Whenever I type Lshift+Y it sometimes gives a uppercase Y, sometimes lowercase Y. Rshift+Y always gives uppercase Y. Rshift+M sometimes gives a uppercase M, sometimes lowercase M. Lshift+M always gives uppercase M. Lshift + Rshift + Y always gives uppercase "MY". Lshift + Rshift + M always gives uppercase "MY".
All other keys and keycombinations seem to work perfect. All the keyswitch mechanisms seem to work fine. Any thoughts on what might be happening here ?
Thanks!

[Weezer]

Could be something up with the sensing mechanism.

[Fkazim]

My recommendation would be to clean the membrane. Could be dirt related.

[HenkHeukers]

Thanks guys. I'll try to clean the membrane, good opportunity to bolt mod the board.