Mike has old type of timer (red sensor, yellow connector) and it has permanent cable. I think that cable is broken (it is really necessary to fix cable in fuselage so that it cannot move during flight). New timers has cable on connector, so it is easy to replace or test. I am in contact with him, so we will see what is problem.
However there are many reasons for such thing, so may be it will be good to collect them. In such case it is good to check from top down:
-defect timer - such erratic run can be caused by broken cable, especially if it is older type with fixed cable, forces in model can move cables and if cable has bad contact, it makes unwanted reactions
-improperly mounted sensor, moving during flight, it can do all sorts of erratic inputs, do not expect that if it was loose once, the next time it will act the same way
-loose cable to sensor, also well fixed sensor can "hear" mechanical movements of cable during fligt, the cable must be fixed also (that is second reason to fix that in fuselage)
-too high sensitivity can cause erratic impulses
-trim problem causing ill movements of model during maneuvers
-problem in power train - for example defect motor winding, bearings etc making motor loose power, power connectors, arming plug etc.