Nitpick: the USSR formed after the treaty of Riga. So no need for that PoD restricion
As for it collapsing, I don't see any counter-revolutionary movement capable to do it - they just lost the war and emigrated by that time. So little chance that a spontanous rebellion like in Kronstadt will be enough to topple the bolsheviks. It would require a foreign aggressor to do it, but there just aren't many available to do so.
Closest best possibility in my opinion: the USSR gets itself involved in a war with Japan or China in the 30's. Trotsky being leader and going through with his permanent revolution policy could make it happen. If it starts in say, 1932, then the USSR still isn't quite ready to take on an industralized state, possibly leading to a stalemate in the far east, Japan being supported in its struggle in a Spain-like manner. This could create the grounds for anti-bolshevik uprising in European parts - especially in the Ukraine, starting a second civil war.