I faced a similiar problem with my Santro. Though it doesn't have a Tacho, the problem was similiar. It lasted for around 4-5 months, during which some mechanincs mentioned that Coolant needs to be changed and some more optimisteic Mechanics offered an Engine Down. I changed the Coolant which dint work and the problem continued. I just ignored the Engine Down problem, as I was sure the engines was working very well.
Strangely, the car was getting stalled sooner with AC on than without AC.
Then, on a trip to Kolhapur, a mechanic cracked the problem perfectly. The problem was of the Electronic fuel pump which required replacement.
I replaced it and there aint any problems after that. it is 3 months now.
I suggest you get your Fuel pump checked once, if thats not yet done.
__________________ Red Santro 1999| Palio 1.6 GTX | Bajaj Scooter 150cc 1999 | |