I'm not very technical so bear with me. I tried to outsource to professional, but a month and several visits later no change. (And of course sorry if this addressed elsewhere, I did look.)

Honeywell R7284U Primary Control
Valliant Oil burner
Tjernlund SS1 Power vent

System goes to Hard Lockout -- No ignition. If I manually reset, it will Hard Lockout at least two more times, but 80% of time fires right up after third reset. Seems to run normally. If a second call for heat comes shortly (a number of minutes later) after shutting down, then it will usually ignite again. If it does not call for heat for an hour or more, when the call comes it will fail to ignite and trigger Hard Lockout.

It calls for heat, starts the ignition countdown (I hear the motor start, the power vent start, and a buzz (spark?)), times out to Hard Lockout

--Have had the basic service and cleaning, replaced nozzle.
--Swapped out primary control for a new one, no change.
--Told I need to completely replace power vent set up, but to me this seems doubtful.
--Draft sensing tube was clogged, I cleaned that out.
--Wanted to clean out power vent motor/fan blades but way it is installed between floor joists not accessible, fan spins freely though. It is a touch noisy, but I did not notice motor feeling warm.

I'm just feeling this should not be 5+ trips from the tech level of complicated. Maybe you can help me narrow down the issue. (Bringing in tech from another service company, but not available until next week.)