My 95 Monte was shifting extremely hard. I found that the vacuum line to the modulator was broken. I repaired the line and the tranny shifts fine. As a result of repairing the line, the tranny now slips until it gets warm. It only does this for the first few blocks, then works fine. The vacuum line runs to a thinga-ma-bob on the side of the upper plenum. Is this a vacuum temp switch?

Welcome to AF. Normally the modulator if working properly and getting vacuum, slippage would not be an issue, is the fluid discolored. A full trans scan will reveal any internal slippage or loss of pressure as a result. What you decribe is most likely inetrnal.

My '96 goes straight into the intake port, no valves at all..

If the tranny works fine when warm, and the oil is at the proper level, not burnt, then this could be something like the thermostatic element, specifically the forward servo thermostatic element. When cold this is suppose to be fully open to allow fluid to help apply the forward servo to apply the forward band. If this element is closed and not allowing fluid to help apply the forward servo, then the forward band does not fully apply and slips until it warms up. When the temperature reaches 32 degrees celcius (90F) the fluid flows easier(is thinner) this thermo element closes, and fluid flows through a single orfice. These elements can sometimes get stuck closed/opened and you can have all sorts of problems.

From what your saying, in IMHO, this sounds like it is your problem. It takes a lot to get to it as you need to take the side covoer off, valve body to get to it. This can be done while the tranny is in the car, but its not fun.