If this effect is only on the first few keys of that bottom row, then it is probably due to one of the traces in the keyboard membrane shorting or not connecting. First I would take the keyboard apart and thoroughly (but gently) clean everything around the membrane, and particularly where the membrane traces attach to the circuit board. Also use a toothbrush to carefully clean around the controller chip on the circuit board - a speck of dirt could be shorting between a couple of pins there. It is unlikely to be caused by dirt in the individual key switches, but if you have the patience you can remove the keycaps and clean inside those switches as well. Some areas can't be cleaned just by blowing.

If none of these steps solve the problem, you could change any electrolytic capacitors on the circuit board - most models have one close to the controller chip.

Rick