heater gone wild - 95 Tahoe

My 1995 Tahoe heater control has gone berserk. Can anybody suggest what may be wrong.
The heater wants to heat most of the time. I removed the vacuum line
from the small vacuum regulator under the hood (on the side of the AC dryer can (?) -- seemed to not let vacuum through to the hat water shutoff actuator), and attached it directly to the actuator, thus shutting off hot water. As a stopgap this seemed to work for a while, but now on the highway the heater goes cold, then hot for while, then cold... I fiddle with the AC and recirc buttons and the temp dial but I don't see any ryme or reason to output temp.
When not spilling heat, the AC does work (I just replaced the compressor clutch, which didn't require breaking open the system, and topped off the refridgerant to the proper low-side pressure.)
Before I replace the small vacuum regulator, can anybody suggest some other checks or part replacements?
Thanks in advance. Jaz (Please excuse the 'burp' when replying)

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