Quoted from HowardR:
Check the switches on the offending reel. One or two of them should be closed except when on zero.
This is that set of 3 switches that are at the top of the score reel. One switch is used to trigger the carry over at 9 to the next digit (closes at 9). The other two are for the reset function (closed everywhere except at 0 like HowardR stated). One allows the score reel solenoid to pulse during reset to make it to zero (this is the one giving you problems). The other is part of the reset circuit that tell the game when all 16 score reels are at zero (this one appears to be working for you or may be stuck open if your game actually finished reset and starts a game but leaves the 10 point reel where it sat).
For the random scoring, it may be a playfield switch gapped too closely. Start a game and then pound on the playfield and see if you get any free scores. If so, try to locate the problem switch. Those star Roll overs are very touchy as far as switch adjustments go. I can't remember if they score 100 or 10. Check and re-adjust.
For the Red and Green Spot targets that control the gate, there are probably two sets of blades on each switch. One controls the gate function, the other would score the 100 points. Check the gap and clean the points on these switches. Also, check for broken wires, not just at the dead switches, but also at other switches that are labeled to score 100 points. One wire would chain around the playfield from switch to switch and this could be broken at a working switch, killing all the other switches down stream.
Rubber Rings usually only score 10 points. So, not sure if there is one on the playfield that would score 100 points to be suspect.
But I almost wonder if the two 100 point problems are related. Look closely at the switch blades to make sure that they aren't touching all the time (which would normally cause a stuck score reel solenoid and bell chime solenoid - assuming your game actually has the bell chime unit installed or connected - many re-import games from Germany were never made with bell chimes). Unless this game is old enough to not use the bell chime unit but instead, has a single bell for higher points, and a bell and clapper that are part of the 00-90 Match step up unit (bell is often missing). If one of the 100 point switches were really close and touching, that one might score when hit but blind the game of all the other ones. During bonus collect, the score motor has switches on it that open disconnecting many features on the playfield from scoring. So, it could go from being stuck on, to opening during a motor cycle, and then score again when the score motor is at home position (between bonus step downs).