Thanks, I will give minrelaytxfee=0.0002 a try. Not really happy with the solution, but it seems there is nothing else I can do for now.
log:
| transactions |memory usage
yyyy-mm-dd-hh:mm|count|size in b|used|free
------------------------------------------------
2015-10-08-11:05|226|658590|167|1880
2015-10-08-11:10|1017|5772645|238|1809
2015-10-08-11:15|1888|12805223|337|1710
2015-10-08-11:20|1777|21506037|424|1623
2015-10-08-11:25|2884|30057925|426|1621
2015-10-08-11:30|3538|35162787|453|1594
2015-10-08-11:35|2934|37325612|482|1565
2015-10-08-11:40|3749|40334966|482|1565
2015-10-08-11:45|4352|42735223|483|1564
2015-10-08-11:50|4974|45271593|481|1566
2015-10-08-11:55|5651|52394742|462|1585
2015-10-08-12:00|7348|70639298|598|1449
2015-10-08-12:05|9006|89281727|813|1234
2015-10-08-12:10|8015|97284225|895|1152
2015-10-08-12:15|17906|102665702|899|1148
2015-10-08-12:20|18155|108387594|897|1150
2015-10-08-12:25|17046|109446477|897|1150
2015-10-08-12:30|18354|119337671|901|1146
2015-10-08-12:35|19031|130917328|902|1145
2015-10-08-12:40|22127|142476766|901|1146
2015-10-08-12:45|23347|156314933|901|1146
2015-10-08-12:50|25156|172886876|903|1144
2015-10-08-12:55|26620|188247301|986|1061
2015-10-08-13:00|26035|198668910|1013|1034
2015-10-08-13:05|27394|210306969|1014|1033
2015-10-08-13:10|28007|222759771|995|1052
2015-10-08-13:15|28833|234213707|993|1054
2015-10-08-13:20|29489|249321745|1019|1028
2015-10-08-13:25|30469|260543586|996|1051
2015-10-08-13:30|31942|274124256|1001|1046
2015-10-08-13:35|31842|286445589|1138|909
2015-10-08-13:40|33439|302956013|1120|927
2015-10-08-13:45|35107|320581461|1178|869
2015-10-08-13:50|36541|334543128|1295|752
2015-10-08-13:55|37615|344550213|1380|667
2015-10-08-14:00|38376|350432866|1407|640
2015-10-08-14:05|36661|352458744|1428|619
2015-10-08-14:10|37265|353242283|1429|618
2015-10-08-14:15|36643|353471887|1425|622
2015-10-08-14:20|37241|354289518|1425|622
2015-10-08-14:25|37869|356291494|1418|629
2015-10-08-14:30|37852|360725573|1423|624
2015-10-08-14:35|37356|364276700|1417|630
2015-10-08-14:40|38402|371417451|1417|630
2015-10-08-14:45|39344|378834124|1417|630
2015-10-08-14:50|38861|382379483|1418|629
2015-10-08-14:55|38577|384160668|1418|629
2015-10-08-15:00|38581|387454439|1421|626
2015-10-08-15:05|39441|391943311|1419|628
2015-10-08-15:10|39237|394802305|1452|595
2015-10-08-15:15|40031|399862537|1452|595
2015-10-08-15:20|39784|404475685|1419|628
2015-10-08-15:25|40495|409814138|1433|614