PEUGEOT 406 COUPE 3.0 V6 24V AFTER 3000 RMP PROBLEM

Hi guys,
I am an proud owner of an Peugeot 406 coupe 3.0 v6 24v - 1998. Since one month I have problem and I have done almost everything to fix it.Since I bought it (from Germany - hamburg at 250 000 km) I start the car it works perfectly on 1000 rpm and wen it warms up it works around 650 rpm, but it starts to tremor and as we checked on the computer it runs around 10-20 rpm difference every time. But that is ok, its 20 years old car and its the best car for me for the budget, I cant afford to change and renew every part.

**** PROBLEMS

1.The big problem starter before 1 month with the torque. When I reach around 3500 rpm the coupe stops for a moment to gain rpm's, speed and after around 5000 rpm its starts again. I have checked and changed almost everything - spark , front coil , gasoline filter, air filter, exhaust and I have run more that 3 diagnostic programs to see the problem but there isn't any prob showing on the pc. Moreover I have checked every sensor and injector (by pulling the buxes out of each one and diagnostics) but nothing, In Peugeot they said to me that they can not detect the prob and for the them the car is ruining good. The Peugeot engine is working really quiet it doesn't touch the oil after 8000km (total quarts 5-40), the antifreeze is full, clear each time I check it and it's not showing any bothering engine, sensor, ignition or gasoline issue and I dont know really
what to do.

2.I have noticed also, that when the temperature goes over 90c only one of the fan is working on the 2nd level and the other is off(the two of them are workig on the 1st level) and it can not cool down the temperature under 90c(I have changed the fan with an working one but the same thing - maybe the prob is in the cables).

Please help with some advice in order to go through the problems of my great pininfarina 3.0.