MY GLOW WORM SWIFT FLOW 80 SEEMS TO WORK WHEN IT WANTS TO?

MY BOILER SOMETIMES COMES ON AT THE SET TIME AND SOMETIMES DOESN'T? THE SAME WITH THE HOT TAP IF THE CENTRAL HEATING IS OFF, IT SEEMS LIKE SOMETHING IS STICKING. I HAVE TO GO AND TAP THE LEFT HAND SIDE OF THE CASING THEN IT COMES ON, SOMETIMES STAYS ON FOR THE DESIRED TIME SOMETIMES GOES OUT. I HAVE REPLACED THE FLOW SWITCH THINKING IT WAS THAT STICKING OBVOIUSLY NOT. WHEN IT'S WORKING IT'S GREAT,PLEASE CAN YOU ADVISE.