It might be a problem of desing, if there is a high current in the power on reset you should add something to limit this current. There could be several reasons why until now it worked. Are you using the same manufacturer for both, micro and 40106, in both cases, with the 80C31 and 87C51? Chips might be working in the limit, in one case inside the limits but out in the other one. Even if it is the same manufacturer, depending on the components itself they may have different ratings. A set of components received all together use to have the same ratings, but could change if you buy another set later.

Javier