92 V6 AX Telstar TX5 - unstable idling speed and momentary loss of power

[cars]

Hi Everyone.
I've got a problem with my 92 V6 TX5. When the engine is warmed up or when for half hour or so and the let the car idle the rev starts 700 rpm. About 1 minute or 2 later it went down to about 600 rpm then 500 until it is very low when the engine is about to stall and very shaky then the engine revs back to 700 rpm. This process repeats over and over. I also experienced a moment (1-2 seconds) of loss power while the car is travelling at 60-63 km/h. This problem is only occurs when the engine is warmed up and it happens intermittenly. I've had it rocker cover gasket both front and rear, cam seals, distributor and timing belt changed the last couple of weeks. I've also had new sparks, air filter and new spark plug leads changed. Does anyone had experienced these problems or know what could cause these problems? I would like to hear from you. Thanks in advance.