Potterton Prima F won't light?

Hello I have a Potterton Pima F boiler that's been giving me problems (2 years ago I had the problem with dry joints on the PCB which I repaired but PCB. Looks good this time. Ive looked into this in thoroughly and im fairly compintant with these things normally (I'm a motor electrician) but I am not a Regestered Corgi engineer by far. At first the issue was I'd set timer to constant, room stat on and then boiler stat to normal position (3 1/2)bthe fan would run constant and at what seemd to me normal speed without cutting in and out but no spark electrode. Looking at the wiring diagram I suspected the APS so renewed this but still no good...! I read a lot of websites saying the PCB gives a lot of issues so renew this but again no joy...! I've removed the front cover checked and removed the top of the flu to check for blockages, I also blew down both red and clear pipe and all good with no blockages. So put all back together and tried again now following the normal start up sequence if found the fan not running now...!!!! So still a little suspicious of the APS and knowing that there's definitely no blockages I removed red pipe to APS and lightly blew down this I then heard a the APS micro switch move then a relay click then sparke electrode operated and then the boiler started, but to my surprise it continued to run when red pipe reconnected or continues to run with red pipe disconnected until boiler warms up to boiler set thermostats position then it cuts out and won't relight I'm starting to wonder if maybe a dodgy NEW APS switch.........Also I'm not sure what sort of pressure and vacuum you should get from the red and clear pipes from the fan but it doesn't seem much (ive been told its not much???) and also once I've got the boiler running and I reconnect all pipes and I watch the micro switch I cannot see it move but I suspect this is very sensitive If you can help in any way I'd appreciate it very much.