It is not how long ago the T-belt was changed, but how many miles/running time since it was changed.
and the quality of the install or parts.
Also be sure that the balancer isn't seperating and loose. I have seen balancers that were loose enough to be moving around while running and give the appearance of timing changing under running conditions. If you say it feels like a misfire, are you sure that it isn't? A bad enough misfire might appear as a timing issue when checking the timing since the RPM won't be steady and with RPM changes timing can/will change due to a misfire condition.
