Tail wheel suddenly sinks into runway and plane ends up vertical - taxiing at 10mph

[tacomasailor]

I've attached three screen grabs

a) iris P-40 taxiing at 8 mph - all is OK - throttle at idle and light drag on brakes - not touching control stick - but tailwheel will not settle to runway

b) down to less than 1 mph - throttle at idle and light drag on brakes - not touching control stick - but tailwheel will not settle to runway - NOTICE Gs have gone to 1.6

c) about 1/2 second after b) - tail wheel has disappeared into runway and plane is going vertical

this happens quite frequently - I find nothing about the problem on the iris forums

Is this a known FSX problem?