bM 2000 & ideal Classic ff370 on off on off issue

Hi,

I have a BM 2000 with an ideal Classic FF 370 boiler, it keeps turning on then off the boiler with like a pulsing sensation, I have replaced the thermostat on the boiler but to no avail it is still pulsing on and off. It doesn't seem to be an issue for water more an issue with the central heating. Can anyone help? I've been asked for error codes but can't find any and am unsure what i'm looking for I can't find the instructions on where to get these in the manual. Any one any idea's of what I can check next. I have searched the forum for similar issues but not found any that have continued after a new boiler thermostat has been fitted. Really need if fixing, i'm a single mum with two young kids and can not continue getting up and down to turn it on and off at the isolation switch. or can anyone recommend an expert local to South Yorkshire I can call in. I rang Gledhill and they said it sounded like a circulation issue but no idea how I can fix it. When draining rads & pump some clear some dark coloured water but no sludge comes out. So hoping its not sludge in the system. Apologies for lengthy text but trying to cover every base. Thanks for any info/recommendations given.

Just to add it was pulsing again and I went into bm to see if any error codes came up just the three lights flashing. I pressed sw1 button nothing pressed sw2 button °69 came up don't know if that's any more use to anyone. Thanx again x