626 Revs drop and dies

My 89 626 quite often packs a sad at me and when driving the rev's drop down ot 1000 or even 0 until it stalls or it sorts itself out...

ie

Driving along at 100kmph, then rev's start jumping up and down, the only way i can fix this is to drop down a couple of gears and rev it up and drop the clutch.

or

Driving at low speeds 40 km or below, when the same situation happens i am un able to keep the car going...

Thanks in advance