Mikesbroken200

Hi, im new to the s chassis scene and just picked up my 200sx ca18det last week. It has been driving fine up until today.
Mods-
*previous owner states Stage 2 horsham chip* ( unsure)
Stock injectors
225 fuel pump
Boost controller 1 bar.
Intake
Straight pipe exhaust
T28 turbo.
Big intercooler.

Issue-
When i start it up it struggles to start and idle for a bit but then works after you give it a little gas. Driving it, it drives fine, boosts okay until its warm, normal boosts are 1 bar at 12-14 afr, idle 16. But when i hold a constant throttle it leans out and goes to 20 afr and putters alot, once it does this it will continue to do this until you shut it off and let it sit for a minute. Once it putters boosting will go to 10afr (mabe less, gauge only goes to 10) and idle will be at 10 or 11. Once it happens its either go or stop, if you hold throttle constantly it will lean out.
-new spark plugs
-new o2 sensor

This has just started happening 2 days ago and i dont really want to melt a cylinder. Please let me know how i can fix it.