Perplexing furnace problem

martin

The specifics Bard gas 90% lowboy furnace 12 years old 95K btu Call was for low heat, furnace was lighting and when blower would come on flame would go out. Installed magnehelic gauge ran about 1" water column of vac off inducer when blower came on mag would spike to 2 max of mag. then settle back but flame would go out. Called tech. help they said jumper pres switch after started, if it kept running bad pres switch replace, it kept running put on temp pres switch of same rating till proper one could be procured. Case Closed NOT Next day client calls from work house was cold when he left, When I hit the door I knew something was not right, got out my combustion gauge it registered 15 ppm in basement and main floor. Shut furnace down and ventilated house till reading was 2. Checked vents and intakehad iced shut from dripping from gutter. Thawed out intake and started furnace burned fine only 20 ppm CO out of exhaust. Called tech about CO in house they said had to be problem in HX to get in house. Sent wife to get new HX at factory about 30 miles from us and proper pres switch, going to replace Satuday. Call up client sat morn to arrange to get into house, said house was cold that morning but was coming up slowly. Went to check with mag on pres switch line inducer would start mag at 1.0 burner would ignite and blower would come on 1 minute later as HX warmed up mag was steadily dropping to .75 where pres switch dropped out flame. About 3 minute cycle. Took off air inlet at furnace same thing took off exhaust at furnace same thing. So I figure HX has got crack opening up when it heats up. My son and I spend afternoon changing primary HX. Get it all installed and same damn thing happens. As it heats up mag reading will go down till pres switch drops burner out and then we start all over again. No tech help on saturday. Funny thing though if I plug air intake at furnace mag reading goes up, will not shut down unit with blocked intake. If you block exhaust unit drops out immediately. I'm now guessing maybe something in secondary HX but am at wits end Any Ideas. All parts looked fine, inducer ect when off to replace HX, Amp. draw on inducer fine no fluctuation can hear no speed change I now have less hair than I had this morning.